Why is this so complicated? The father did NOT pay the loan but instead gave the cash to the son, who did pay the loan. Son gets to deduct the interest. Now, since they are both legally obligated, and if the interest is say $5k, I don't see a problem with Dad making half the payments and son making the other half (with Dad's money). In that case, they could both deduct $2500. That might be a tactic for next year.
I have always had a complaint about the limits on the student loan interest deduction. For single filers it starts to phase out at $65k AGI, double that for MFJ. These are the people who make too much to get grants so rely on loans. Make less and you can get Pell grants; make more and you can presumably afford the cost of college. But make $65k and you can't get grants and can't deduct all your loan interest either. Just another one of those items congress passed to make it look like they were helping voters out with college costs but wrote rules denying those people the benefits. (Exactly like the credit for the elderly--how many of those has anyone actually done?) And don't get me started about why it seems no one actually saves for their kids' education. The student loan situation is crippling our young, they can't move out of Mom's basement or start a family, total student loan debt is larger than credit card debt, blah blah. Easier to complain about the high cost of education and loans than to put away even a modest amount every month when children are young to ease the burden when the time comes.