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My question is can I claim him as a dependent on my tax return? He is under age 24, was in school 5 months in 2016, and lived with me for half the year. But since he had a significant income on his own, that makes me think I am not entitled to claim him as a dependent.
If he provided more than half his own support for the year, you can't claim him as a dependent. You can use the worksheet on p. 16 of IRS Pub. 501 to calculate it: http://www.irs.gov/pub/irs-pdf/p501.pdf
See my answer to your other question.
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