To be fair, some people are indeed reasonable and tend to want to work things out, come to agreement and THEN go to a lawyer, saving considerable amounts of money and heartbreak, and paving the way for some method to move forward. My ex and I did that twenty-odd years ago, with a house and two tween-teen kids. Was it hard? Yes. Was it reasonable? Yes. Total cost for BOTH of us: $2,000 legal fees to document what we’d decided.
It can actually be done.
But, to the tax point, yes - he will need a written agreement before any deductions can be made. Some spouses WILL agree to do it one year at a time, so as not to lock themselves in. Perfectly acceptable. The alternative is to stop paying and wait, banking the money against the day repayment will be needed.