Here's a thought that I hope a tax expert will chime in on. From the information you provided, your son doesn't qualify as a dependent. You might be able to claim the home your son is living in as a second home that you and he bought together. The mortgage deduction allows you to deduct the interest on your main home and one second home. You don't have to live in the second home at any time during the year. This will allow you to deduct the you on your main home and your portion of the second home up to the IRS limits on mortgage deductions.
Translated from Pub 936:
If your son received the Form 1098 showing the interest paid, attach a statement to your return explaining this. Show how much of the interest each of you paid, and give the name and address of your son. Deduct your share of the interest on Schedule A (Form 1040), line 8b, and print “See attached” next to the line.
Similarly, if you're the payer of record on the mortgage, deduct only your share of the interest on Schedule A (Form 1040), line 8a. Let your son know what his share is.
