You do not need to provide him with receipts. It is his responsibility to help pay for his children, and portions of that money should be included in HOUSING COSTS, FOOD, Nursery fees, Clothes, medical expenses, fun, etc.
As with any parent, he needs to pay child support for the children because they are HIS kids.
I am aware that men feel like child support is unfair to them - but what about the kids? This money is FOR kids. For supporting kids.