Try an intervention type thing perhaps? Get seom of his (non boozing/doper) friends together and whatever family is willing and put him on the spot. Try and get him cut off from booze and weed and he may get better. If it continues to escalate or you feel like you can't handle it, it may be time then to seek professional help. Intervene early before it gets to bad, and the problem is a whole lot easier to fix.
Oh, and never do anything you don't want to do. Go to the party if you want, but don't drink if you don't want to. I didn't drink for my first college years and it didn't inhibit my partying or fun having at all. Hell, half the fun I had was messing with the drunk people's heads
If he gives you crap, just let it roll off, he'll get over it.
Conformity sucks, never do anything just cause others are doing it.