Reconnection with an Ex that ran off. What should I do?

I recently got in contact with my Ex-wife that ran off with our son two years ago. Our son is three now and I want to be in his life but, I know nothing about him. And she acts as if nothing happend or as if she did nothing wrong. I am told that my son now has severe seperation anxiety, but I would like him to be able to come and live with me for a while so that I may get to know my oldest son. How do I deal with Her and get my son to know the father he has missed out on for so long.