He's insecure

So all my boyfriends ex's have cheated on him and he's always so insecure about himself, doesn't think he's good enough for me, and constantly thinks I'm going to cheat on him( I would never cheat on anyone)

Even if I talk to a guy friend he gets insecure and thinks I'm going to leave him.

If any of you could give me any advice to help him with his self confidence or help him forget about the cheating in the past