What Kitty said about your mortgage is true. Like I mentioned you can get the house signed over to one or the other but it really means nothing. You have to get yourself off the mortgage period and that means refinancing. I agree with everyone. You need a lawyer to make sure everything is correct and also to protect yourself in the future. Bptoo said it best, you own nothing to anyone except you and your son. It's okay to want to be civilized and but you don't go out of your way to make it nice for him. Ask yourself, would he do that for you? Not that you have to be spiteful but there just is no need to put yourself out especially for some other woman.
I am curious also if you have talked to her. She as a woman should understand how you would feel. She has no business coming into your home moving and changing things. That is your home!!!!
Take care and try your best to relax and get some time for you and your son. (I know easier said than done)
Heidu