Can I tell my roommate he can't complain about being fat because he eats like shit and he's lazy?
Do you think that will motivate him?
This is an oversimplification on the subject, but it will work on him.
Well, it is in his control to change so he shouldn't complain. It would be one thing if he was okay with it but he should take the responsibility to change if he wants to.
I am willing to work out with him.
That is nice of you to want to help him out and I can understand why you were upset. It is hard to see people complain but do nothing to change things.