Dear XXXXXXXX,
They will take out less if, when he gets married, he completes a new W-4. He would change his marrital status and his number of exemptions on the W-4, and submit it to his employer. The employer is required to take out taxes based on W-4 information. In the absence of a W-4, the employer must take out at the default rate of single with zero exemptions.
http://www.irs.gov/pub/irs-pdf/fw4.pdf
Tax Preparer
GPHR Cert; U.S. Treasury Tax Advocacy Panel appointee