If it is earned income (i.e. W2 job), then the filing requirement for a child being claimed as a dependent on your return would be $6,300. So, he would not be required to file. However, if there are taxes that were withheld from his paycheck, then the only way to recoup that withholding is to file a return. So, it may be beneficial to file a return in that case.
IRS Publication 501 - Filing Requirements for Dependents (pages 3-5)
https://www.irs.gov/pub/irs-pdf/p501.pdf