I can't say for sure if he's sleeping with any of them or not (I think probably not), however...
There are so many guys out there who wouldn't do this and recognize it's disrespectful to your relationship to keep naked photos of your friends, even if the girls send them to him (or even more likely their ex-boyfriends are passing them around).
It's your call, I suppose you can give him another chance and see how it goes, but I'd rather date a guy who understands it's not right. I'd even be able to respect a guy who may not think it's a big deal, but has the courtesy to let me know he was sent these pics, because he gets that it might be an issue for me. This guy clearly does not (or knows and doesn't care, which is equally bad and inconsiderate). P*rn is one thing, but people you actually know? I'd feel uncomfortable with that, and if he's upfront about these things he'd know that, and hopefully he'd respect my views on it or let me go my own way.
The other problem is your trust in him is already dissolving, and without that it's almost pointless to carry on a relationship. If you were the type who could look at those pics and not be bothered your boyfriend's jerking off to photos of your friends, my answer would be different, but I don't think you are. You'll find yourself snooping more, checking up on him, feeling insecure, getting clingy, etc. You'll turn into the bad guy down the line, and it's just not worth it.