Most Helpful Guy
Let sleeping dog lie is my advice. He hid important things from you just as he is doing to this other chic. He has proved he can't be trusted to be monogamous in a relationship and won't man up and admit it when he had done wrong.
Most Helpful Girl
He's playing both sides of the fence. If he could move on that quickly with someone new (yourself) while on a 'break' imagine what a relationship would be like with this man. At best, you were most likely just a rebound to him. I say cut your losses and don't talk to him anymore.