Child Support during separation period

I have been divorced since October 2017. Before that I was separated and living at different addresses for over four years. During this time, my ex-husband has been paying child support monthly. However, this agreement was made without going through the court or any other legal system. I made some research and learned that child support is not deductable. My ex-husband, however,insists that he is going to write it off in his taxes. Can he claim this money as alimony (even if our agreement was child support) even though we were legally married until October 2017? What will be the consequences if our taxes differ in this matter?