Me and my boyfriend have been together for 22 years off and on have 4 children together he has 3 from other relationships he has been with me and another lady at the same time and he had been dealing with her for about 6 years and has been back and forth. He recently was incarcerated for a year I waited on him did not talk to anyone cheat, and when he gets out he constantly brings up an incident that happen 2 in half years ago, which I did not have sex with this man he was just an old boyfriend but since then he has been involved with other females and has hidden things from me when he was locked up that he says he was not doing, like sending cards and letters to his baby mama, and when he got out that's when I started to see things, my question is, is the reason he does not trust me is that he is steal cheating.

'is the reason he does not trust me is that he is steal cheating.'
YES
Cheaters always assume that others cheat. It's also a good way to keep you constantly trying to get his approval. You spend all your time proving that you aren't cheating, instead of concentrating on what he is doing. It's Classic Cheat tactics on his part.
(As for your title question - don't bother to try.)
Time to dump this man. Plan quietly. Gather some money to save. Find out where you can go, if you can't kick him out. How old are your kids?

I guess I have to wonder why in the >>> do you care. Why did you wait while this loser was in prison, why are you wasting your time on him ?