Alimony is only tax deductible if it is court-ordered, not if it is voluntary.
If this is ordered as part of your divorce, then look on the page for Deductions and Credits. If you don't see Alimony, look for a link for "show me more options" or "let me choose what to work on." Or you may find that the most common deductions are highlighted at the top of the screen and other items are at the bottom if you scroll down.
Alimony is tax deductible to you and taxable income to him, so you may want to remind him you are reporting this deduction. If he doesn't report and pay tax on the money he receives, the IRS will send him a bill for unpaid taxes.