The above answer is not entirely correct for two reasons. You can be considered a medical dependent of your boyfriend even if you are not a tax dependent if:
(1) you lived with him all of 2017.
(2) he provided more than half your support.
If this is true, he can deduct the medical expenses he paid on your behalf even if you are not his dependent for tax purposes. But this is still subject to a threshold (7.5%, not 10% as suggested in the other answer) that only medical expenses in excess of 7.5% of his AGI can be deducted, and he must also itemize deductions.
