Well you had it in your head that since he was staying there must be something still there that's fair enough. It's normal to feel jealous/upset because your life is forever changed. You just lost your security/family unit/familiarity so ya it would be a bit of a shock to the system.
The reality is, you turned a blind eye to what would happen eventually. He's moved on, and you foolishly didn't prepare for it.
IMO him saying it might have made a difference if you said something is horse manure. He's just pushing this back on you. He's the one that ended it, and he knew you wanted him back. So if indeed there was a change of heart he would have made that move.
Life will be harder at first, feeling dejected, but you have a responsibility to yourself to make your life better for you and your son. You will eventually find much happiness down the road. Don't let this eat at you, you needed this kick to the pants to really start living/enjoying life.