I agree with all of the above posts.
You can't trust him. He's proven that.
It sounds like his ex is around to stay too. She still wants him, and being that she's his child's mother... if you stay with him, she will be in your life forever.
My husband has a child from a previous relationship, and his mother is a CONSTANT source of agitation and drama in our lives.
Follow your gut instinct. I would bet that it's telling you to run. It will hurt for a while, no doubt, but it will be 1000% better than the wasted years that may be ahead dealing with the rollercoaster he will no doubt put you through.