If you are paying the interest on the loan, it does not matter if the loan is in your father's name. This is a relatively common occurrence. As long as you have ownership interest, it is yours to deduct. There is not special form. You can fill out the 1098-INT section as if it was your mortgage.
I would encourage you to consider amending your prior years' returns and taking the deduction as well. You may be able to get a substantial amount of money back.
