Only income tax can be refunded, not Social Security or Medicare tax. That's why his refund is zero. If no income tax was withheld, there is nothing to refund.
If $2,160 of wages and tips is your son's only income for the year, he is not required to file a tax return. Since he is not eligible for a refund, there is no reason for him to file.
It's good that no income tax was withheld. It saves him the trouble of filing a tax return to get a refund, since he doesn't have to pay any income tax.
