my sweet little boy will not admit he has a "problem"

couple of years ago, my husband and I found out that our son who is now 17yrs old has been using drugs, mailly marijuana for the past four years. We could not belive that our son had been doing it for so long and we had not notice it. His grades were not the best but we simply assumed that he just needed extra help at school so he got admitted into the special education program at the middle school, but still his grades did not get any better. My son has been a very respecful, polite and kind kid, never complained or shows any sign of anger so his problem with drugs never ever crossed our minds. Until, the principal found a small amount of marijuana in his backpack when he was in 10th grade. Immediately he was suspended from school and it was a wake up call for us. Now, for the past two years it has been a nightmare for my entire family, we have tried different methos and nothing seems to work. We have seen several therapist, Psychologist , doctors, couselors. We have tried to be tough with him at home, we took everything away from him (no TV, no video games, no ipod, no computer, etc) and still he went back to smoking, he ran away for four days, finally he came back home (druged). We put even more restrictions, made him signed a contract that basically he had to agree to stay clean if he wanted to leave at home (this was my husband's idea). My son was clean for a month or less and again went back. I have cought him spoking few times in his room. He basically keeps breaking our rules. it breaks my heart watching how he is destroying his youth, like i said he is a very kind, respecful and bright kid, he is so charming and charismatic that everone that meets him really gets impress by him. On the other hand, I am so desperate that I just don't know what to do or say anymore. I feel like this a night mare that just does not end. My husband keeps saying that as soon as my son turns 18 he has to stay clean or leave our house. I don't agree with him and as a consequence, we constantly fight about my son, we are so close to get divorced. My son is now at a different school and he is doing better, he has a girlfriend who does not approve this smoking habit. I have talked to my son about this whole situation and he keeps asuring me that he does not have a problem, that he only smokes once a while and that he is not addicted. He also says he needs it because it helps him with his creativity, he likes to draw and paint (he is very talented), but I don't see him painting or drawing anymore. I have also caught him laying to me, and constantly asks for money(not much but he does constantly). My friends keep telling me that my son eventually will grow out of it, but I am so scare that this is only going to get worse. I am so afraid of loosing my sweet little boy too soon..