Did he get a W2 or a 1099Misc for it? If he got a W2 he doesn't have to file except to get back any withholding taken out in boxes 2 and 17.
Be sure on their return they check the box that says they can be claimed on someone else’s return.
Filing requirements for a dependent
https://ttlc.intuit.com/community/children-dependents/help/do-i-need-to-file-my-own-taxes-if-i-m-a-d...
