There are several tests for claiming someone as a dependent.
Presuming your boyfriend is a U.S. citizen and knowing he resides with you takes care of two of these. He is not a relative (hopefully, as that leads to other complications), so the only question is whether your relationship breaks any laws
(against co-habitation, etc.), since the tax law contains provisions prohibiting obtaining a tax benefit
to support an illegal activity. This will satisfy another of the tests.
The remaining tests are support and income. You must have supplied more than 1/2 of his support, counting amounts he may have received from his family and spent on his living expenses or public support monies (Social Security, Unemployment, Worker's Compensation
, Insurance proceeds or Welfare). If you paid more than 1/2 of his support (food, clothing, shelter, medical expenses
, etc.), then you would pass this test.
The final test is the income test - did he have $ 3,050 or more of income for 2003? If he did, you may not claim him as a dependent.
You can file an amended return for 2003 by contacting your tax professional or just by getting the forms
from your public library.