If this was an alimony agreement executed on or before December 31, 2018, then it is deductible to him and taxable to you. You would enter the amount of money you actually received from him in the alimony section on the income page. Don’t make any allowances for whatever tax you think he might or might not have paid, just report the actual cash money amount that he paid you.
If this alimony agreement was executed on or after January 1, 2019, then it is not deductible income to him nor is it taxable income to you. However much he sent you, you don’t report it.
If you think he sent an incorrect amount, you will have to take that up with the divorce court.
