If he is not your dependent, he may deduct it, taking the attitude that you paying it is actually a gift to home and he has effectively paid it.
There is one other possibility ( a special rule),. If your son (or other close relative) cannot be your dependent because he had more than $4,200 of gross income, but otherwise would have qualified as your dependent (basically that you &/or other family members provided more than half his support), then you can include in your deductible medical expenses any qualifying medical expenses you paid on his/her behalf just as if he was your dependent.
