My ex husband got a tax refund. Does that mean he didn't pay enough in child support?

My ex is supposed to pay a percentage of his income in child support after federal and state taxes (and a few other things) are deducted. We exchanged tax returns, and I noticed he had a significant tax refund. Could this mean that his withholding was higher than necessary? Since we calculate the amount he pays based on his paycheck, I'm wondering if it's possible the net income we're using for the child support payment is artificially lower by having higher withholding. Would a percentage of his tax refund be eligible to have the child support percentage applied to it?