Butts are not chocolate soft serves. Yes, some poop can come out, but not that often or that much if you’re aware of your body. That said, it will happen at some point. It’s inevitable. Don’t make a big deal. Simply ask to change condoms. Or, if you don’t mind, afterwards, take off the condom, wrap it in a paper towel and throw it out without him seeing. Then go wash yourself off. You don’t need to tell him it happened if he didn’t realize. There’s no reason to embarrass him.