Thanks for all the helpful advice/suggestions. He's quite mature and well able to handle himself in any tricky situations that might come up as he's been overseas enough to know the routine. He's also quite comfortable doing the flight himself.
We've given him lots of independence from an early age and he's never disappointed us. We went to HKG last October for a week and left him on his own with a friend; they alternated staying alone at our house and then at his friend's house with his friend's parents. It turned out to be a good combination of independence and supervision. We're doing the same arrangement when we go to Rome next month.
I also don't worry about him being in AMS, as he's quite level-headed. And he's already seen European tolerance up close. He and I did a driving tour of southern Belgium and northern France last year. One morning we got up early in Ypres (Ieper) and he saw some local high school students having a beer, smoking, and playing cards at a pub before heading off to school. (A bartender in Brussels later told me there is a lot of concern with teenage drinking in Belgium, but no one seems to do anything about it. Whole other topic.)
The discomfort comes from the parental side, mainly my wife. In many ways the world now is so much more worrying than it's been in a while, although obviously that's not stopped us from traveling as frequently as time and money will allow. I'll go just about anywhere at a moment's notice as I'm a real travel junkie (I'll have well over 125K actual flight miles this year). Wife less so.
So bottom line is wife says no and we can't convince her otherwise. I'm not going to push it as he's our only child and if she's not comfortable with it, so be it. After 18 years of marriage (anniversary today!) we pretty much understand each other's feelings about most issues and our negotiation patterns are very well established.
He's young and will have many opportunities. We have a family trip to Australia and New Zealand already planned and I may take him to Vienna pre-Christmas. So don't feel too sorry for him.
Thanks again and best regards to all.