Friends with Benefits Goes Wrong...

So me and this guy have been fwb for a few months now. We like each other but we can’t date so we decided to keep our relationship as it is. We had an agreement that if one of us were to get into a relationship, that this whole friends with benefit will be over. This leads to me finding out that he has a girlfriend through Instagram and it seems like they just started dating recently. My suspicions were confirmed when his friends told me that he has a girlfriend (they don’t know about us). I kind of question him but he didn’t mention a girlfriend so what should I do? I want to call this friends with benefits thingy off but I don’t know how to confront and tell him.