I am in love..... but............?

Are you sure he's only smoking pot? That actually makes people eat more, so I'm surprised he lost a lot of weight. The thing is, this is what he's doing with his life right now and it's not likely you'd be able to change it. You can try to hang out with him more and suggest other activities to get his mind off it, but it's no guarantee that he won't just hide it from you and tell you he stopped.