Versys, you have to pay what you have to pay, but it sounds like your ex is living off us tax-payers, fraudulently, and you are therefore being hit by the CSA.
I'd suggest you report her with as much info as you have, and try to re-form a relationship with your son. he's old enough to be supporting himself and would be entitled to claim benefits in his own right, but I would say if you CHOOSE to give him (and not your ex) any further maintenance, good on you.
I wish you the best of luck.