Why does my ex act like this?

My ex-boyfriend and I went through a very messy break up at the end of last year, he broke up with me but we then got together afterwards and kissed goodbye before I went overseas, promising to meet up when I got back. Anyway..i came back after the summer but we didnt talk to eachother as we were both over it, but one day I texted him to see how he was and he went ballistic at me and told me to leave him alone. Since then we agreed to not be friends, but everytime I see him (we have mutual friends) he freaks out and just walks the other way. I think he just feels awkward as he doesn't know how to act around me, but what can I do, as i'd really like to be friends with him again at some point.