"Do what you want, but be ready for the consequences" works every time.

oh and "Friends respect each other's will" fits well too.

Having said that, isn't having you do drugs also a will of his that you ought to respect? My reason is all twisted now...

its cold down here fam ~

Don't go to the parties, especially if you succumb to peer pressure and people jeering you on to just try it once. Don't let him charm you to going either. Stay firm to your beliefs because getting into that stuff is not the way.
When you're not a minor you can enjoy alcohol (it's overrated though. Seriously. The novelty of being able to drink wears off after a while), but the drugs? Don't. Just don't do it. It's not a path I recommend following.
You can continue to enjoy his company, but if he keeps pestering you about it I don't think he's going to be worth your time.