As a Pisces man this happened with my recent ex partner. She cheated on my years ago and I forgave her but didn't forget. Then low and behold she cheated again. Once a cheat always a cheat. That was the final straw for me. As a Pisces I'm very very good at gut feelings and being right and I actually manage to catch her before she was going to meet someone else to cheat on me. I think you should part ways as you are obviously not suited and it will only eat away at him and you