Caught my bf sexting

So over the past week and weekend,I had a acquaintance of mine from school say she had something interesting to show me. Turns out that my bf had been sexting her via Kik. And at first I didn't believe it and my fear kicked in and I checked his phone while he was asleep...I didn't find the Kik app but I found nudes of him and screenshots of a girl who sent him videos...honestly I don't know what to do. I confronted him about it and he seemed more embarrassed and ashamed then he was sorry. I don't know if I should break up with him or ask for some space to think...the worst part of the whole thing is a few weeks earlier he gave me a promise ring,now it feels tainted. I could really use some help here ladies! Any advice would be greatly appreciated.