Op - what happens when your DS is 18, and an adult, in presumably less than 2 yrs? Will you try and stop him going out and having a drink, staying at friends, just in case he has too much and chokes on his own vomit?
You say he is an A student, thus in just over 2yrs, he will probably be at uni and getting plastered in freshers week - how will you know then if he drinks too much, vomits, chokes?
I know there is a difference between being 16 and 18, but it seems your reluctance springs from your anxiety, and that wont change in 2 years, you need to learn to manage it and trust him. Yes he will almost certainly get drunk and vomit again in the next 2 years. Yes, he could choke, but unlikely. He will however, learn his limits, and what to drink/not to drink, if he is ill through quantity or mixing. Hopefully he will then be more sensible at uni.
Staying over at after prom is a rite of passage, both mine did it, neither got drunk to the point of being sick, as far as I know. They are now 18 and 20, drink but are sensible.
One of DS friends was drunk at a pre-drinks session prior to a 6th form social. Got picked up and taken home rather than going to social. I dont think she has been that drunk since then, over 3 years later.
My DS is at uni, just finished second year. I started off wanting Whatsapp messages after nights out, making sure he was safe etc, but I have learnt to cope with my worries and hope he is sensible and with a group of friends that watch out for each other. I realised that I cannot project my anxiety onto him.
As a pp said, if you are too hard on your DS now, he will only lie to you over next 2 years and you will have no idea where he is. Better to know where he is and be available to pick him up if he feels ill.