Some teenagers start drinking earlier, some later. It must be so dull for teenagers stuck in lockdown, so I'm not surprised they might want to add interest to their walks/outdoor meet-ups. My starting point though would be to contact the parents of some of his friends to find out how they feel about it - it's always worth investigating the claim "everyone else is allowed..."! Then you can all be consistent in your approach, whether that is banning or allowing in moderation.
When my elder son at a similar age wanted to throw a small party at our house (pre-covid), we had a frank discussion about alcohol and I came to the conclusion that I would prefer him and his friends to drink beers I'd provided (smallest bottle possible and lowest alcohol) rather than get fake ID and try to buy it themselves from an off-licence, which they seem to manage incredibly easily and were clearly determined to do. And for him to feel he could be honest with me about drinking, so we could keep talking about it and I could keep an eye on how much and what he was drinking. He's now at university and he has turned out very sensible around alcohol, which is a relief. He has said how much he appreciated me listening to him and being willing to compromise at times.
I think it is a good sign that your son feels able to ask you this, so I hope you can find a good solution.