Been there and did take him back, the first time, the second time, the .... Well you get the picture. I know couples who have been through this, stayed together and developed stronger relationships. Others like myself, dealt with a repeater and simply couldn't handle it. Everyone is different. It's not like he's been out there having a LTR or even intercourse. This may well be a one shot deal and when he understands how much it hurt you, it may not ever happen again. Only you or the two of you can decide what to do now.