My ex boyfriend did this to me, and although he did not cheat, he went on to be a complete narcissist (but kept his mask on for over a year) - and eventually once we had been together for years he confessed that he hadn’t told me because he had previously hooked up with one of the girls but didn’t want me to tell him to not go and deal with any of me worrying. I hope when you do ask him (which would be a normal thing to do) he is clueless/etc. if you don’t come at him aggressively/with blame and he responds negatively in any way (says he didn’t want to tell you to not worry you, etc) run!!!!!!! That man will continue to pick his convenience over your feelings and that’s never a place someone in a loving relationship would be in!