If he doesn't think it's his child, he needs to prove that with a paternity test. If he truly can't afford the payments (say if his income changed), he can ask to have them lowered.
Otherwise, well, kids are expensive. You have an option of being with him or not being with him, knowing that he has a legal and ethical responsibility to pay out this money every month and support his children. You have the choice of staying with him or leaving him, given what you know about his financial responsibilities. But you can't change the fact that he has these responsibilities.