I don't think the state cares if he can afford it or not. If the kids stay with you most of the time he should be paying something
Well I have quite a few years experience here....
Its done off Federal Standards, each state must set their calculations to comply. The worksheet calculator can be a bit cumbersome however it takes into account columns for both income and child related expenses for each parent. Those include who provides medical as well as who pars for any required childcare.
This is then bounced against the number of days each parent will have the child physically living with them and a calculation is made on the total support required. This is then broken down by the percent calculation based on actual living with parent days to determine what percentage of the total support required belongs to each parent. Support can be no lower than that amount for the state to remain within guidelines unless special circumstances exist. It can be higher though. There is also a top end cap for high earners and that can't be breached unless one party can show an "Accustomed Lifestyle" will be lost (i.e. the kids always went to private school before the separation). In my case, $1049 a month.......Cap was just under $1200 a month and I hit it when I was earning a bit more in SoCal.
This is the area that can be gamed though to get a calculation that works for both parties. Some will agree to split the days in a way that helps one party or the other and not really the days they are actually staying with those parties. The more you move to 50/50 and the closer the incomes are, the more the calculation is Zero or net zero (less than $100 a month). Not implying anything here by the OP, just pointing out the system.
In the previous divorce, I had primary Physical Custody, we pretty much split the days even. My income then was just under twice her's, I paid medical and daycare as wel, and she would have owed a little bit. I didn't need or want it so we adjusted the days so her owing me was right at $100 a month. State won't get involved in mandatory withholding at that amount (differs by state), so I wrote her a receipt for $1200 paid the moment we left court (so she had to pay nothing that year).