He will always be out of money when you need and have the control to say no, or you are spending too much. It is just a control issue. He needs to pay $XXX.xx amount each month, carry the child on his insurance, and he can pay for half the child care if you really want him to. I had this happen at my child care center and it did not work, one or the other was always late or just didn't pay, that effected the child and the mom ended up having to find other child care. I can't tell you how many times I heard, "I don't owe any late payment fee, I paid you on time", "well, your child care is not paid, you have to pay it and a late fee", "You'll just have to talk to him/her..." The child care center has to be paid regardless and you may as well just plan on paying it and if he pays then you skip a week or something.
As for state guidelines, you can look up child support guidelines online and they are based on minimum wages. Even if he is a seasonal worker he will still have to pay several hundred dollars a month.
Make out a budget for him. Show him just how much he's trying to say he'll be willing to pay. For example, it looks like a LOT of money when you add up all the things you listed. He would most likely pay less through court ordered.
I would go ahead and get the child support on a court order, that way if he doesn't pay they will start taking it out of his paycheck each pay-day. It is much more humane to have an agreement in writing with a judges signature on it so you don't ever have to argue about it again.
All my grandkids that I am raising has child support deducted from wages before taxes and other deductions. They get some kind of tax break on it but we still claim the children on our taxes as our Dependants. I know that money is going to be there every week, except when they are terminated from employment. That has happened once or twice but they always let me know if they aren't working so I can budget for it.