S.M.
I believe it is fair for you to stop paying her. But if she wanted to be bold she could sue you for support.
What you are paying amounts to child support and child support is considered neutral, neither of you have to claim it or get to claim it. The others are right in saying that you don't have the legal right to claim him because he doesn't live with you. She only has the right to claim him if she supports more than 50% of his needs. She is supposed to divide the cost of her house and utilities by the # of people living in her home. That is the amount that is his portion of those. Then she can add in food he eats and any direct expenses she pays. BUT, the amount you give her should come off of what she spends. If she isn't paying more than half of his upkeep, then she really can't claim him either.