If he gets a W2 he doesn't have to file a return if it is less than the Standard Deduction 13,850, not because of the 4,700 or 4,200 limit.
If their only income is W2 and under 13,850 they do not have to file a return except to get back any withholding taken out. If he got a 1099Misc or 1099NEC you have to file it as self employment income no matter how small the amount.
If he does file his own return then HE has to check the box on his return that says someone else can claim him.
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...
