All debating whether pot is addictive or not or if it leads to other drug use aside, the fact that you asked if your boyfriend's habit was a problem suggests that it is to you. If you accepted it you would have no need to discuss the issue. I suggest you fully examine your feelings and then decide if it is a problem or not.

I agree that alcohol is a worse problem but the fact of the matter remains that alcohol is legal, pot is not. Your boyfriend can end up in jail with a criminal record which will drastically impact his ability to find and hold a good job as well as his reputation in the community. Are you willing to risk that happening? You may end up falling from grace with him.