IMO you have already "done enough" for him money wise and it can be someone else's turn. Someone who is not already raising his child with no assistance from him and does not have any sort of emotional investment. There is help out there for him, you are simply the most convenient option.
My exah is always trying to get me to "help him" with money even though he does not really need it (and actually is supposed to be paying me child support!).
My answer to him is "I gave already, this bank is closed".