You would be claiming him as your qualifying child. The relationships listed in IRS Pub 501 that meet the test for a qualifying child include:
Relationship Test
To meet this test, a child must be:
-
Your son, daughter, stepchild, foster child, or a descendant (for example, your grandchild) of any of them, or
- Your brother, sister, half brother, half sister, stepbrother, stepsister, or a descendant (for example, your niece or nephew)
of any of them.
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https://www.irs.gov/publications/p501/ar02.html#en_US_2016_publink1000220886
