insecure boyfriend help.

My boyfriend and I have been dating for a little while now and don't get me wrong he's good looking and all but he's SOOOO insecure.. he thinks I cheat on him and mess around / flirt with other guys while I'm with him he just accuses me of all this and gets upset when I tell him otherwise because he thinks I'm lying. I just don't know what to do or how to help him? Do any of your boyfriend's go through this?