Change in withholding.
The reason why your son's tax withholding is higher is because of the expiration of the "Making Work Pay" credit. Because of the credit, the withholding tables were changed to allow people to get more money back from their paychecks. In turn, the credit was suppose to make up for the difference.
Now that the credit has expired, the withholding tables changed to reflect the loss of the credit.
However, you son should see the amount of Social Security withheld decrease by 2%. Depending on how much he makes, this may be less than the changes due to the "Making Work Pay" credit.
Did he end up owing last year? If not, and you don't expect him to owe this year, he should change his withholding status to exempt. That way, they do not unnecessarily take out withholding if he's not going to end up owing any income tax. That should make him happy.