Most Helpful Guy
As you said, people say all sorts of things in the heat of the moment, but given the background of your story, I can see why he might say that. I think that it's not so much that he's implying that you're sleeping around, but he's describing how he feels you're treating him. You're having this relationship with him, but you won't give him the commitment and affirmation that he feels he deserves given the state of your relationship. This is the doing "whatever with whoever" that he's talking about.
Anyway, the guy is clearly hugely insecure and while I don't think you should cave and tell him something that you don't really mean, maybe you can find a way to assure him that you like him and care for him? Although to be honest, there just may be nothing you can do about this guy. Maybe it's a little old school male of me, but I feel that men should keep insecurities like this to themselves rather than dumping them on their partner.
Most Helpful Girl
Sounds like he said he will fuck who ever he wants