I'm not sure what to do..

Ok well.. I've been friends with this guy since the 5th grade, and this year we're both seniors, we're both born on the same day, so you could say we know each other pretty well.
recently his sister told me hes back on drugs, I know he can do so much better, but his reasoning for taking it is losing his dad.. the thing is, he lost his dad in 2007, i know its not a long time, but the fact that he keeps using him as an excuse to do the drugs just seems wrong to me. I know what it's like to lose a dad, hell i watched mine die right in front of me, and i'm aware that everyone deals with their problems in their own ways, but this isn't right.. is it? i want to confront him about it, i just want to talk to him tell him theres people out there that really care, i told him once before, that i'd always be there if he needed someone.. he lies constantly however, looking straight into my eyes and tells me he hasn't done it for a long time, he just took a drug test, it came out postive. I don't want to put any pressure on him, i'm trying to be the best friend i can be, but to lie to my face everytime.. just what should i do?