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If your son earned only $2800 - it is unlikely that he provided more than half of his own support - and he is full tile student under 24 - he is your dependent.
Please refer to the IRS publication 501 for the definition of qualified child - http://www.irs.gov/pub/irs-pdf/p501.pdf - see table 5 on the page 11.
There is no way your son will receive a refund for that $146 that were withheld.
The only difference is on his tax return - assuming he will file the form 1040EZ - http://www.irs.gov/pub/irs-pdf/f1040ez.pdf
On box 5 - If someone can claim you (or your spouse if a joint return) as a dependent, check the applicable box(es) below and enter the amount from the worksheet on back.
and the amount on the line 5 would be $5450 instead of $8,950.
That might affect your ability to file electronically - in this case you would mail a paper return to the IRS - but will not affect his refund or your qualification to claim him as a dependent.