if you already told him that there is no smoking whatsoever and he knows how your parents feel about it.... fck him, kick him out. Actually it'll be better to give him an ultimatum(sic). No more smoking period or he is getting kicked out. The friends over the house all late and sleeping on the furniture would have made me kick him out already. Smoking pot is ok, just respect other's wishes.
FYI, I smoke 24/7 and I hate cigarettes. I find it no problem to abide by other people's rules on smoking weed.