If he made $400 or more he has to file a tax return with Schedule C. If his total income is less than $13,850 he will not have to pay any income tax, but if he made more than $432 as a coaching assistant he will have to pay self-employment tax.
If your son is your dependent (or a dependent of anyone else), when he files his tax return make sure he checks the box that says someone can claim him as a dependent.
