1. You can deduct interest if you are a legal or equitable owner, even if you are not listed on the mortgage itself. (I'm going to ignore the concept of "equitable owner" because you say you are a legal co-owner.
2. You can deduct interest on your main home and one second home. If you want to designate your son's home as your "second home" for the interest deduction, that's fine.
3. However, your maximum mortgage indebtedness for this deduction is $750,000, so that might disallow you depending on the size of your own home mortgage (if any) plus this mortgage.
4. You can your son can split the interest based on how much each person pays. However, because of the high standard deduction, it will usually be better if one person claims it all and the other claims nothing. (For example, you could say that he pays 100% of the house and your support pays for other things. Or vice versa.)
5. You can't combine to claim more than the total interest paid.
6. If the 1098 is in your son's name, you would enter the interest as if it were on a 1098 and check the box for "someone else's name is on the 1098." There is an old IRS instruction to attach a written statement to your tax return, but Turbotax does not allow attachments for e-filed tax returns. So I would just report the interest and see if they send you a letter asking questions.
7. You can also deduct property taxes on any property you own (located in the US). It does not have to be your residence. However, there is a $10,000 cap on all state and local taxes, so again, it may not help you to split this deduction.
