"But getting back together means that they are a couple again."

That's not a TaxPayer Unit descriptor.

Him making More or not, also isn't part of this.

Scenario example: He pays her alimony. They decide to move back in together, but not remarry. She continues to pay all the bills she paid while single. They Split the mortgage and related housing costs. She covers all cost for the child. Etc.

Follow what the IRS states, and then what they do is applied to what is required. As Lisa described: it's your Due Diligence to file properly.

It's pretty much everywhere for reference:

https://www.hrblock.com/tax-center/filing/personal-tax-planning/what-is-head-of-household-status/

https://smartasset.com/taxes/what-you-need-to-know-about-filing-head-of-household