Yeah, the claiming him as a dependent may affect the Premium Tax Credit for health insurance However, see the bottom section of this comment box.
From what I've read, the IRS assumes he qualifies as your dependent (full-time student under age 24) unless you prove otherwise. The fact he has not changed his mailing address would seem to indicate he has not permanently moved out.
As for support, his own non-scholarship money would need to pay for over half of his total expenses, so that doesn't seem to be the case.
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So from you are saying, I interpret that he probably qualifies as your dependent.
However, just because he qualifies as you dependent does NOT mean you need to claim him. If you do not claim him, his income will NOT count for health insurance purposes.
For his tax return, because you are ABLE to claim him, he can not claim himself (which probably won't affect his tax return for 2018, as the exemption amount will be $0).
On the other hand, claiming him *MIGHT* benefit you for health insurance purposes (in your case, I suspect it won't though).
The Premium Tax Credit is based on percentage of the Federal Poverty Level. So if you do NOT claim him, only your income counts (which would increase your health insurance credit), BUT you would have a smaller family (which would decrease the health insurance credit). If you DO claim him, his income would be included (which would decrease the health insurance credit), BUT you would have a larger family (which would increase the health insurance credit).
However, as I mentioned before, because his income is rather large, I suspect the benefit of a larger family size would not be enough to offset the negative affect of adding his income. But you probably want to try it both ways to see what the best result is.
