He cheated

Some random person messaged me saying my SO was cheating on me and sent me sc of the messages between him and a girl. The girl sent several pics and he sent one. She asked if he had a girlfriend and he said “we are working things out” (which wasn’t true) I called him crying and he came to my house crying as well and said it would never happen again and that he’s a piece of shit. No I didn’t leave him. It’s been 3 days now and I can’t stop thinking about the messages and asking him why? We’ve been together for over 4 years. Wants ironic was that he was always scared of being cheated on... Is it true that once they cheat they will do it again?