I think an important thing to gauge here is his relationship with drugs. Is he always high? Does he test his drugs to make sure they don't contain anything dangerous? Does he get them from a trustworthy source? What I'm suggesting here is that experimenting with drugs does NOT make someone a bad person, and that it's in fact very natural to try them out and use them recreationally every now and again, especially as a teenager. HOWEVER, if he is very irresponsible with drugs and seems to be dependent on them, then stay away from that relationship. You want to be his partner, not his therapist, and that is what you may become if he does have a drug addiction. I think you should definitely inquire about why he lied to you as well, perhaps not in a confrontational way but in a way that aims to understand his motives.
Something I should clarify is that if the prospect of being in a relationship with someone that uses drugs in any capacity makes you uncomfortable, then this isn't the relationship for you and you would certainly be better as friends.
Ultimately, it's entirely up to you to decide how to approach this situation, but please don't do either of these two things:
1) Ostracise him and treat him as if he's a bad person for using drugs
2) Make excuses for him and engage in a romantic relationship if he's addicted to drugs