In a situation like this, I would ask myself "What good would it do?" There are always what-if's (what if his dad mentions it, what if he nobody mentions it and you bring it up, etc.) Not to be harsh, but would you ever tell him that he was almost aborted? You were at the clinic and in the room and changed our mind? What good would that do? It might be a hurdle worth waiting for or maybe his personality/maturity is one of knowing ahead of time would be better.
I have a friend that was in the exact situation as you. (She did have an abortion prior to this pregnancy and she said she thought it would be easier to give up a baby than to abort again. In the end, abortion was easier for her b/c it was done so early, so she never felt the baby move or had a delivery, etc.) Her daughter is now 9 and she doesn't know. Her and her ex both agreed to never tell her and they threw out the photo's of the couple they picked out that were there for her birth.
Now that your son is here, is your ex behaving more appropiately? Would he agree not to mention it to him?
If you do decide to mention it, you might want to try and explain it to him that you loved him so much that you thought you couldn't provide properly for him, that you wanted to give him the best life possible.
If it were me, I don't think I would mention it. I would take the risk of him not finding out b/c there is also the chance he never would. Also, if you wait, he will be older and if he does find out, might be able to comprehend the situation and understand it better.
I wish you luck in your journey and in your decision on what you think is best for your family.
One of the hardest things in life is being a parent!