he has no choice in this. if he is the father he is obligated. In some cases fathers are responsible enough to do what they should be but in other cases, as it seems here, they want to pay as little as possible. As long as you enable him to make the choice himself he will continue to opt out of helping out more.

Yes. If it is established he is the father, and she is with you the majority of the time, get a court order, have it calculated, and if necessary they will take it from his paycheck if he doesn't pay up what is owed. I think that's mostly standard across all the states, assuming you are in the US .