Note that in this situation, if he uses itemized deductions, he can probably deduct his mortgage interest, but he can't deduct property taxes that he pays.
Mortgage interest can be deducted by a legal owner or a beneficial owner. Beneficial owner means someone who owns the property in all but name -- they pay the bills, mow the lawn, perform other maintenance and upkeep, and generally take on the responsibilities of ownership, but are not on the deed. Assuming you are paying the mortgage from his money, then he is really paying it, and he can deduct the interest.
However, property taxes can only be deducted by the legal owner, if they pay the taxes. There is no allowance for beneficial owners when it comes to the property tax deduction.