What you claim at work on the W4 doesn't have to match your actual tax return filing status. It's just to tell your employer how much to take out. You will get credit for all the withholding on your tax return. I'm married but claimed Single on the W4 to have more taxes taken out.
He can fill out the W4 how he wants. But on your tax return at the end of the year you file either as Married Joint or you each file Married filing Separately. Joint is the best way to file.
