Just ignore him and the next time he does apporach you and become annoying that you don't like him because he's "fat" tell him it has nothing to do with that, it's the fact he's so negative all the time and you don't need to be around people like that. Block him online and when you see him outside where ever you go and he happens to be walking past you, say "hi maybe even a How are you?" but keep walking as you say it. Or go out with head phones on and listen to music that way if you do see him you can easily ignore him and move on.

^^^Don't have your headphones up too high, though, if you do decide to do this...you should always be well aware of your surroundings, especially in the vicinity of someone you feel may do you harm (he might not take being ignored lightly).