Sneaky boyfriend

Okay so my boyfriend started messaging this other girl (met her once at party, don't even live in the same town) and we broke up over it and then he came crawling back to me and I said I would only get back with him if he stopped messaging her and he said "of course anything for you" and it's been almost a month and he's started again but he thinks I don't know and I need to confront him about it but not sure I can bring up that I know about it? (Did some digging to find out)