The music majors thread is full of parents of young musicians. They will know.
Typical suits with 40" jacket come with 34" pants. If there is an option to buy suit separates, that can help, but the jacket would still have to be altered for the slimmer waistline. (Dress shirts often have the same problem, although the rarely found “athletic fit” ones may help.)
Note that the waist is typically assumed to be 6" smaller than the chest (this is called the “drop”). Sometimes, “portly cut” suits with 4" drop are offered. Rarely, “athletic cut” suits with 8" drop are offered.
Thank goodness a friend steered me to a discount suit place at our mall. 179.00 out the door (w/alteration on hem of pants only) with a shirt, tie and belt included. Is it the best quality? Probably not, but given that son is likely still growing and filling out, I’m happy with how it looks and how well it went through the cleaners.
@HImom, it may be a local thing, but no one wears cummerbunds or fancy (ruffled) shirts here anymore. The tuxes here are now all plain shirts and vests, and they are showing them a lot with long ties instead of bow ties. So in that case, they really do look like black suits but for the satin lapel and trouser stripe.
@colorado_mom, I agree with you that the buy one get one deal is great at Men’s Wearhouse! We bought my HS senior son a suit, and my husband got one as well. Since you get one of the same or lesser value free, and my husband was willing to spend more, our son ended up with a pretty nice suit for an 18 year old! We had started out looking at the cheapest suits for him, but when we realized what the deal was, no reason not to let him get a Ralph Lauren, right?
@NYMomof2-we bought suits for my boys (for oldest S’s wedding) at Jos Bank. They are all quite slim, but not as tall as your S. They bought “slim fit” suits, if I remember correctly.
dadx, Do you mean that the length of the jacket should match the place where the fingers attach to the palm? Or a little shorter than that?
I’m leaning toward trying to buy an inexpensive suit. I had DS try on a navy blazer which is a 36R, and it seemed to fit, with enough room to play.
This suit is in stock in that size at the local Men’s Wearhouse - any opinions?
Two button, side vents, plain front pants.
I’m looking online at Jos Banks, but they seem to have very little available. I wonder whether they have more in the stores?
Our oldest got a lot of use from the MW Pronto Ultimo suit. Not as good a deal as our youngest’s new suit, but we were new to suit buying for young men. He wore it 3 times…about 100.00 per use, whereas youngest has worn his 179.00 ensemble 3x already and will wear it at least 3 more times by graduation. Gotta love that deal 
“Typical suits with 40” jacket come with 34" pants."
It depends on the brand. Some Hugo Boss 40R trim suits come with 32W pants. (Thank goodness!)
NYMom, that suit costs only 3Xtypical tux rental fees - good deal, IMO. On the model, it looks just fine. If it fits well, no one is going to scrutinize the label.
Right, it only needs to look good from a distance, fit well, and have enough room in the arms and shoulders for movement. I’d be happy if he got to wear it 3 times before outgrowing it!
He has a very nice white oxford cloth button-down collar shirt from LLBean. He usually wears it with khakis for jazz band performances. I have the feeling that he shouldn’t wear a button-down collar with a more formal suit? I don’t like the shirt the model is wearing in the Pronto Suit, but DS should have a shirt with a collar that doesn’t button, right? And not oxford cloth?
I’m looking at Lord and Taylor now. They have a lot of suits on sale.
Mr. Who Never Shops gets his shirts from Nordstrom Rack. He just bought 3 of them*, because he is off to a big meeting, and some gremlins stained a couple of his good shirts with tomato products.
I always get a kick out of the tag that simply says “Dress Shirt” No shi(r)t! 
IMO, iron-free and trim fit is the way to go.
*You know what I meant
Those are nice shirts, BB, and I do know what you mean!
FWIW, DH did compare prices with Mens Wearhouse and Macys before going with Macys, which was a much better deal.
OP, how old is your S? How many more music performances will be in future? Can he use suit or a tux for prom if he’s still in HS? If he’s headed off to college in fall, there may be formals in his future…
After two tux rentals for prom in HS, I bought him a tux (same price as the 2 tux rentals) senior year. He used that same tux in college for the various formals, fraternity events for 4 years. Turned out to be great investment. He tried on and liked fit of RL at Macy’s…I found same one online with shirt for $300. Tux rentals in our area run about $180 a time. He just rents or buys different ties/bow ties/vests as occasion dictates. l
Kohl’s is good for inexpensive men’s dress clothes.
I was really happy with Men’s Wearhouse Buy One/Get One. The salesman spent a very long time ( more than an hour) with us fitting and finding ties and shirts to go with his suit. Little brother,15, got one too. My son is 6’ and slender. I think it was “athletic fit.” The salesman put them both in a size larger and had the suits tailored in about four days. When they grow future tailoring is free. Son #1 got both the job and was admitted to the program he was going to interview for.
In college a friend was a music major. He had a tux and used to pick up spare cash playing at temp events. I can see a musician needing a black suit.
They’ll guarantee it within a week. If you’re really insistent they can have it done in a day. If you go into a Men’s Wearhouse they’ll measure you and get the right size, but if you wanted to buy online and then take it to a store for alterations, measure his chest with a tape measure under his arms, then over his arms. His size would most likely be the circumference of his chest under his arms in inches (so if his underarm chest measures 40 inches you want a size 40) but if the difference between under his arms and over his arms is more than 10 inches then maybe go up a size or two. 6ft tall is right in between a regular and long, but you should get a long because it’s easier to shorten than lengthen.
Pronto Uomo is a Men’s Wearhouse house brand. It’s should be fine. It’s not a super fancy suit but it’s affordable and decent.
I’ve spoken to a friend whose son did this same event a couple of years ago, and she recommended renting a tux at a local place I hadn’t known about. They have great reviews online. They aren’t open today, but I will take DS there right after school tomorrow. I’ve looked at what they have, and there are some styles that are less obviously tuxes than most. He can wear a formal white shirt and bow tie for the piano solo and a colored shirt and regular black tie for the band (the boys are wearing black suits and ties and different colored shirts). This will be the fastest and easiest way to get him something for the concert that is now less than 2 weeks away, at a time when he has a lot of schoolwork, he needs to practice a lot for the performances, and he is preparing for the March SAT. My plan is to buy him a tux or formal black suit in about a year, when he should be closer to his young adult size, that he can wear for prom and in college and possibly after. He does play two instruments and he performs a lot (solos, large bands, various small ensembles, pit bands) and I’m sure a tux will be a good investment. At this point he is still growing so fast that I periodically notice that he’s shot up again. And he is really skinny. In a year he will probably have filled out.
Thank you all for your help! I feel much more able to navigate the options now that I know what features to look for, and how to figure out what will fit him.
gosmom, I like the RL suits/tuxes I’ve seen online.
csfmap, I didn’t think of Kohl’s, although that the one store I have dragged my sons to when they needed khakis or dressier pants fast. I didn’t realize they had suits and tuxes, but I looked online and they have a very large selection in black, including a lot of suit separates, which might be the best option. (And they have a big promo for gray suits called, Shades of Gray - ugh!) I will run there if the rental thing doesn’t work out.
And if/when I buy something I’ll get a second pair of pants, on the suggestion of someone who PMd me, because it’s hard to match blacks later if pants wear out.