There may have been a mistake in the return, and the IRS just corrected it.
It could be the Earned Income Credit (EIC), or another credit. He should have (or be getting) a letter explaining the change.
If not, the only recourse is to contact IRS directly at 1-800-829-1040. Give yourself a lot of time, they're swamped now! And unless you have a Power of Attorney on file with the IRS, he's going to have to be the one who calls.
