I would definitely read up on what to do in an emergency from Talk To Frank, it is handy to know for life in general.
I would say to your son that after Christmas you want a proper conversation about what is going on.
That gives you time to decide what you want to do.
Now, next bit is hard, I could not tell mgbparents anything and actually it made me more vulnerable when I was using drugs.
For example, if i was in a bad way I wouldn't go home, I would walk the streets or stat with strangers to avoid seeing my parents and this lead to other bad things. (Another topic, another day)
If you can cope with seeing him in a state, I would make sure that he knows you are there to help him and he can always come to you.
Personally I would rather deal with a wasted teenager and keep them safe than find out something bad happened to them because they didn't feel as if they could come home.
Do you have any one to talk to in real life?