Thanks for the suggestions. I am going to ask son to ask girlfriend what she suggests, but I want to make sure he doesn’t break any unspoken rules and is respectful.
Son should DEFINITELY exercise his duty-free allowance & buy a 1 litre bottle of booze in SIN. The manufacturers specially package 1 litre bottles just for airports. All alcohol is expensive in Singapore
Maple syrup is also very expensive in asia. If his hosts don’t want it, I’ll take it! 
Just make sure the parents don’t have serious objections to alcohol for personal or religious reasons. There are Singaporeans who abstain or don’t keep alcohol in the house due to admonitions to minimize or better yet, avoid any consumption of alcohol to adhere to their strict interpretation of various religions including evangelical Christianity*.
- Some take such a strict interpretation that they're effectively teetotalers.
OP here. I’ve been told by my son that I need to stay out of his business about what he’s taking, etc. I will ask him to check with the girlfriend to see if she knows of any expectations, but I’ve got to back off a little. Other son would appreciate advice. This one not so much. I just hope they don’t talk bad about his parents if he does the wrong thing 
Smart kid. At least he won’t be pushed around in life or by future wife and in laws.
Just give him one more important piece of advice. If he decides to bring maple syrup, put it in a ziploc bag. It ALWAYS leaks.
GMT, it is possible that it leaks because the blasted TSA opens it and sniffs it to make sure it is what it is. Too easy with screw caps. They always do this with the honey we bring from HI!!! Never fails. I had no idea they did it, but once a nice TSA person left a hand written note on the little form they put in your luggage explaining that they had opened my honey… And I naively thought that the plastic film sealing the jar just popped off.
I didn’t know the TSA open our luggage, no wonder I don’t carry luggage.
Yes, TSA opens luggage. And when they do, they leave a form in there that acknowledges they’ve opened your luggage and examined all your underwear. (Just kidding about the underwear).
The maple syrup in my bag (my kids love maple syrup) tends to leak in the low air pressure regardless of whether TSA has opened my bag. And it leaves a sticky, gooey mess.
Well after these comments maple syrup definitely won’t be on the gift list (that he won’t let me make).
Yes, TSA opens your luggage and leaves a note. BUT when traveling abroad, you are not dealing with TSA once your baggage leaves the country. Our luggage was opened on every leg of our trip four years ago. No notes were left, and some things were actually taken (bags of candy, for example).
Thanks for the heads up @thumper1. Once on a trip from Hawaii my son brought a skim board. The TSA stuck the very sticky sticker right to the part of the board that should be very smooth. We never did get it off.
Once on a family trip in the Caribbean, and my son brought a whip (a real cow whip, and he paid $160 or so for it). It was in my bag, because only my husband and I were checking bags. They called my name to go into the area where they checked bags. I was really stressing about how I was going to explain that whip. Fortunately it was my husband’s bag they went through.
Last year for Fall Parents Weekend, I brought S2 a Halloween prop he left at home: a latex fake bloody severed hand w realistic looking bones jutting out the wrist. I deliberately packed it in my checked bag for TSA…
OP here. I just dropped my son and his friend from Singapore off at the airport. He had two of his friends from Singapore here, and I was really glad to get to meet them before sending my son off. They are traveling most of the way together (friend wound up on different flight from Japan to Singapore, but thought it was the same), so I’m less worried. For those who are interested, turns out my “gut” was right, when I was concerned about staying with girlfriend’s family. Let’s just say there was some mis-communication somewhere along the way, and he will be visiting the girlfriend, but NOT staying with them, at least for the majority of the trip. He will be staying with his male friend.
Again, your kid is smart. Trust him.
Son survived the trip, and is now somewhere within the Washington DC Area Metro system. No caning for chewing gum violations or any other troubles in Singapore. He had a great time. Next trip, LA to work for the rest of summer.
Thanks to all who provided advice.
Thanks for the report and checking back in. I’m glad everything worked out well for him. Good luck on the next venture!
No daughter-in-law?
No daughter in law. She won’t be going back to his school, so I suspect that relationship will be over soon. But they really liked each other, so it’s kind of sad.