If he goes back in for a modification, you can ask that the court "impute" his previous wage to him.
The court cannot make him work at the higher wage level, but if he's voluntarily underemployed, it can certainly assess him the support rate he would have paid.
Now, if there was a serious downturn in his industry and no good jobs are available, then he might have a plausible argument. But usually someone making that wage can usually find another job making that wage, even if it's not doing the same job.
Kids are the priority. The court doesn't like to reward folks who voluntarily take lesser-paying jobs, in the hopes that it will reduce the CS.
Otherwise, you would see a rush on fast-food jobs!