He is not mature enough yet to have a real relationship. He clearly wants to sow his oats. You should tell him that he has put you in a difficult and horrible position, since you have to see Emma, and you like her. For now you will just have to deny all knowledge should she find out- after all, its just a secret you have to keep. But I would encourage Ben to do the grown up thing and finish with Emma in order to get it out of his system, OR stay with her and give up these other girsl. Either way, he needs to make a decision.