[abousoun]

whatever your relation was with his parents, they should know ... minnik aw min ghayrik mish fer2a ...

second, you should talk with him and try to see what's wrong ... eno obviously he has some sort of problems which lead to this drinking and drugs issue ...
so once you can get to know what's wrong, you can help him with that ...

try to invite him to somewhere far from those friends, and near to some friends both you know and do some sort of "intervention"

if all didn't work, report on him and his friends if you're sure they are taking drugs

Thank You ...