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You can claim your son for 2004 if you provided more than 50% of his care throughout the year, his income was less than $3100 and he cannot be claimed by anyone else, including himself.
It sounds like he had a full time job and that he did not live with you for very long in 2004.
If I have read this wrong and you have been advised that you CAN claim him, your son will need to amend his return and not get the exemption for himself. This will mean that he will have to pay money back that was refunded to him in error when he claimed himself. If he no longer has the money, he can ask for a payment arrangement to repay the funds.
The form that he needs to amend his taxes is a form 1040X. It can be found on the IRS.gov website. He will need to amend his state return as well. He may owe money back for state taxes, too.
If you have questions about this, please let me know so that I can instruct you further.