A dependent's earned income is never entered on your tax return.
If the income was earned as an employee where a W-2 was received then he does not have to file a tax return if that is his only income. Unless he wants to file to get a refund of the taxes withheld (if any), then he would enter on his tax return that he can be claimed as a dependent.
If he earned the income as a self-employed person receiving only cash or a Form 1099-MISC then he is required to file a tax return since the income was greater than $400. He reports the income and any expenses on a Schedule C for self-employment, part of and included with his personal tax return Form 1040.
