My boyfriend is worried that ima cheat. :(?

My boyfriend is always saying how he thinks im going to leave him for other boys or cheat on him and stuff. He knows about one of my ex's and my past and always thinks im going to run of with my ex or cheat. I ure him that i wont and he knows that i have never cheated before. He like always brings it up saying oh yea i bet you'd rather be with the other guy right now and stuff.. He says he trusts me but he just can't stop thinking about it and bringin it up. I really dont know what to do :( I always tell him how much I love him and he always has my friends telling him how much i talk about him and stuff. Like he believes that I really do love him but idk..he just always has bad thoughts.... :(:(:(