Denial. And not being willing to talk about it, if he's a man. Men aren't supposed to have weaknesses, and depending on how far your character's bought into the whole "cult of masculinity" thing, this more than anything could be the cause.
Also, with addictions its a slow slide. If you want to realistically portray it, show the casual side of substance use; he's getting high, having a good time while still remembering to function as an adult. Then slowly take that functionality away from the character. And all the while, he's denying he's got a problem. Which might be true at first but would become less so as time goes on.
Just my two cents.