Does the dad have to pay for a child he has with me ,if he is now married with another child with her and two others that are not his?they are on benefits-

It's his child. Having married someone else does not change that fact. One can't go around spreading one's sperm and just decide to not be responsible about it. (Nor can one go around accepting someone's sperm and not be responsible for it. And this one sounds like a real winner.)

Of course, the total legalities will depend on where you are. Go to Google and put in "child support" and your location.