trust issues and reassurance

ok so my boyfriend and i have been together for 4 months and we’ve known each other for a LONG time. Every relationship he has been in his gf has cheated on him. He is still having issues with trust. Likes he is complicated ok hahaha. So he says he trusts me and obv he loves me and stuff but he still has thoughts sometimes and he thinks I’m gonna leave him for someone else. And i always try to prove to him that i won’t EVER cheat on him and reassure him. It works but then his doubt comes in again and i have to reassure him. I have no problem with telling him how i won’t ever do that but i hate that he feels that way. Has this ever happened to any of y’all? How do you prove and earn his unconditional trust and never make him have any doubts? Plz help i love this man more than anything