Most states DO include the other children the person is responsible for in the calculation of CS. NJ is one of them. See the NJ child support guidelines, page 10 section a:
Other Legal Dependents of Either Parent: http://www.judiciary.state.nj.us/csguide/app9a.pdf Also, NJ
does set an age for child support to terminate, which is 18 unless there is post secondary eductaion, or some other extenuating circumstance, and then other rules apply (Page 22 Section 18) Also, unless he has an order from Child Support Enforcement, he CAN send payments to her. However, if he was ordered to send it through them then he won't get credit for anything he sends to her directly.
My advice would be for your fiance to meet with the division of child support enforcement and try to hash out credits for what he has paid. Hopefully he's kept records or he's out of luck on that one.
Secondly, he should ask for a modification hearing after your child is born. The new caculation should take into account his new child, and his new income.
Beyond that... something in his story is not adding up. If he's been sending her money through the court then there must have been an order in place. He can get a copy of it, if he doesn't have it, by contacting the division of child support enforcement. He also would have been notified if there was a hearing, and he would have had to provide income information.
Demand to see a copy of the CS order. How he treats his first child is a very good indicator of how he will treat his other children - so I think you have a vested interest in this. You need to know what the real story is. You're having this guys baby, and you're about to marry him. Time for full disclosure sweetheart.
Also... I do
not recommend garnishment. It's too difficult to coordinate if an adjustment is made, and you could end up paying more than your supposed to for months. If that happens, you won't get it back.