NTA.

When infidelity occurs, it causes a major break of trust in the relationship. Typically you need counseling - or at least some form of transparency to ease back into trusting another individual. It appears you were either allowed on his phone, or snooped. If you snooped, this is more of an ESH, because you need to work on transparency and communication skills with your S/O. You need to recognize that without being on the same page, the relationship will not work (or be stuck in some annoying loop).

look, you know he’s a serial cheater. i assume you are hoping he changes & you are different than the rest. the only way to become different from the rest is for him to be meeting you halfway on the trust issues. if he will not meet you by allowing you to ask what is xyz or see his phone, and that’s what you want to know he’s no longer cheating, then that will always be an issue. otherwise, you have the choice to turn an eye to his behavior and allow it to continue or obviously, break up. a lot falls on what he will do to improve the scenario and fix the trust.