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Your son can be your dependent if he meets one of these two tests:
Qualifying child: Your child can be a qualifying child at age 27 if they are disabled, they live in your home more than half the year, and if they provide less than half their own support. Their support can come from many sources, and you don't have to provide more than half, as long as they pay less than half their own support. However, for income tax purposes, "disabled" means they are unable to perform gainful work. The IRS does not consider sheltered workshops or occupational therapy to be gainful work, even if he is paid, but the ability to work a part-time job for regular minimum wage is almost always taken as proof that the person can do gainful work and is not disabled for income tax purposes. If his permanent home is in your home, and he travels occasionally, that can be considered "temporary" and he still is considered to live in your home. But the independent living in New York may defeat the residency test.
Qualifying relative: Your child can be a qualifying relative if he has less than $4700 of taxable income, and if you pay more than half his support. For this test, he does not have to live with you, but the income test is firm, and you must provide more than half his total support (considering his wages, SSI, Medicaid, and anything else).
If your child does not qualify as a dependent, you can still deduct medical expenses you pay for him (but not other support costs) if you provide more than half his support, and the only reason he doesn't qualify as a dependent is that his taxable income is more than $4700.
