Did your son have a lawyer? I ask because child support is based upon a formula in NC that takes into account the incomes of each parent, the number of children, the cost of health insurance and the cost of work-related child care. If he consulted with a lawyer at the very beginning of the separation, the lawyer would have been able to calculate his child support obligation before your son signed a lease on a new place. It's always best to at least pay for a consultation with an attorney before taking on new financial obligations after a separation. As many have said, the same income the parties had during their marriage now has to stretch to cover two separate costs of living. There's nothing mysterious about this. Everyone gets poorer when there is a divorce.
And, OP, you probably won't get any sympathy about your son not having time or money for a social life. He WILL have plenty of time to himself during the week while his children are with their mother. His evenings will be free of child care responsiblities. He won't have to help with homework, cook dinner for the kids, pack lunches, lay out their clothes for school the next day, bathe the children each night, read to them before bed and log it into their reading logs for school, take them to soccer or ballet after working all day, etc., etc., etc., He may not have a lot of money to afford a social life and his every other weekend will be taken up with the children but he will have a lot more free time than the children's mother, especially if she also works.