If your son's earnings were reported to him on a W-2, and if that was his only income for he year, he is not required to file a tax return unless his income exceeded $12,200 (2019). However, he would have to file if he wanted a refund of any excess withheld taxes - but he wouldn't be required to do so.
As @xmasbaby0 stated, if his earnings were reported to him on a 1099-MISC, he'd have to file if the earnings were $400 or more.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
