If he is a US Citizen then he may also be having to pay tax there, despite living overseas. US citizens have to file tax returns even if they don't live there.
Quoting the IRS web site:
If you are a U.S. citizen or resident alien, the rules for filing income, estate, and gift tax returns and paying estimated tax are generally the same whether you are in the United States or abroad. Your worldwide income is subject to U.S. income tax, regardless of where you reside.