Is this "son" still treated as your dependent for federal income tax purposes, or not? If not, then he should be getting a refund of most of the federal income tax that's withheld, and should qualify for an EITC (which is a fully refundable credit), which would mean he would have a net negative income tax liability. The payroll taxes will still be levied, and aren't refundable (and, to be perfectly honest, will never come back to him as social security will no longer exist by the time he gets to retirement age). There is no federal sales tax, although there are federal excise taxes on cigarettes (thank you liberals, it's now several dollars a pack), as well as on gasoline (I'm assuming he drives and pays for his own gas).
No, he's not my dependent. He's already filed his taxes, the figure I gave was what they actually kept. He is single, can claim no dependents himself.
I would figure his total tax outlay, as I said, at up to 1/5 to 1/4 of his gross, say $3000, by the time all the different taxes are added up. Even if he had custody of his kid and could claim one dependent and EIC, he would still pay SOME amount in, even at $14,000 a year.