If you own the house together, the IRS allows you to use any "reasonable" method to split the interest deduction between you. The reasonable method has to tie back to who is making the payments. If all the money for the mortgage payment is coming out of your paycheck or bank account, the default reasonable method is going to be you get the full deduction; he gets none.
If you want to have him receive some of the deduction, you need to have some of formal agreement showing that he is paying a portion of the mortgage, even though it is coming out of your bank account. If he is paying your some sort of child support for instance, you'd want written documentation that part of that payment is to cover mortgage costs.
Once he is "in the game" so to speak, the simplest way divvy up the interest deduction is proportionally. If you are paying $1,000/month, and you are saying that $400 of his child support is for mortgage, you would get 60% of the deduction and he 40%. But you don't have to do it that way. If you wanted to be creative, you could split the deduction up in ways that help your overall tax situation (but I assume that's outside the scope of what you are asking.)