He might be your "Medical Dependent." Here is the IRS wording on this situation
You can include medical expenses you paid for an individual
that would have been your dependent except that:
1. He or she received gross income of $4,050 or more in 2016,
2. He or she filed a joint return for 2016, or
3. You, or your spouse if filing jointly, could be claimed as a dependent on someone else's 2016 return.
