If he’s legit 5’6, he’ll be somewhere around 36-40 depending on how slight/stocky he is. At that height, 140 pounds will put him at around a 38(short). Under 140 likely puts him at 36. Above 147 or so puts him closer to 40.
I think you’re going to have difficulty finding the right selection. Make sure you take your time and don’t settle. I still say start at Nordstroms, although they’re probably geared for older bodies and may not have much in the shorts below size 38 or so.
My advice is for someone who works in finance or top end law or similar activity. If you’re in another area or field, take a look around at those 15 years older and heed the cues. Steve Jobs probably didn’t own a suit. :)…but what he did own fit correctly.