My boyfriend doesn't trust me..what do i do?

So.. i really want to go to PCB with my best friend over the summer but my boyfriend is really upset about the idea and doesnt want me going because he doesnt trust me. This is why..one time i went to a party behind his back and someone called and told him to come. so he did..and he thought that i was going to hook up with this guy there but i really didnt. and he believes me now but hes still scared i will cheat on him. He has been cheated on before and i understand whys hes so scared...but what can i do to show him hes the only one for me, and that i would NEVER do that..and show him that he can trust me?