I can see why you are worried about him.
Looking at it from his point of view he has spent 16 years on the social periphery, not invited to parties, no close friends. All of a sudden someone has taken an interest in him and his Mum is trying to put a stop to this!
We know that he is vulnerable to exploitation. He is likely to go in excited and hopeful of a new phase in his life, pinning a lot on this, and find it hard to have any objectivity or sane reflection at his disposal.
You need to be that sane reflection for him, coupled with explaining your concerns to him and saying how pleased you are he has found a friend. If you actively try and ban this, he will start to be underhand about what he is doing and where he is going in the future.
Ask him about this lad, how old he is, who his parents are, will they be there, would you like to invite him round sometime, please ring if you have any concerns and you will come day or night etc. etc. ....
Honestly it is all you can do. You can't chain him up; and you need to try not to alienate him.
It is hard - we are older and wiser, we know how naive we were at that age and the things we got up to - we know what the risks, both physical and emotional are - so we worry.
Make sure you tell him you love him and trust him.
I had 3 DDs - all now adult - I have been in your position - it is just the same, male, female, gay, straight..... with the added risk of pregnancy - be thankful you do not have that worry!
Gently let him go with your blessing and an understanding of your worries and an awareness that these come from a loving place.