As far as the support test is concerned, you can claim him as your dependent if you furnish over half of his support. The money his mother gives you would affect that, assuming it was earmarked for his support. So, not considering that, you would have to provide over half of his support.
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When you add someone as a dependent, we'll ask a series of questions to make sure you can claim them. There may be other tax benefits you can get when you claim a dependent.
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