His past left him with poor self image?

My boyfriend is one of those muscular but kind of (he can bench 245) but tiny (he can wear a size 28 pants but prefers 30) and short, like 5'4. I told him he is hot but he says no, i need to be thinner. He thinks of himself as a loser. And has said before that he deserves someone like misty, but 10 years younger. Misty is a trashy bartender that works with him who always flirts with him and shes 28 and hes 18. He has zero self image because he was a...loser. His whole life was going to work at a bar (underaged), making money, buying weed or alcohol, getting wasted/blazed, and doing it again the next day, selling eventually to keep money in his pocket. He doesnt do that now. Hes starting his senior year with intentions of going to college to be a history teacher. How do i show him hes so amazing when he just ignores me and gets upset when i try to.