In your situation, a letter from his brother might have some weight. The brother could confirm that he’s not been around for years, that your children are not included in his family’s events and holidays, that he takes steps to avoid you and the children.
It is really impossible for anyone but the FA officer to know what a school wants for the waiver. Last year there was a poster on CC whose ex did pay court ordered child support, but she still got a waiver at St. Olaf (and I think several other schools). We don’t know what her divorced agreement called for but whatever it was, St. Olaf accepted it and granted the NCP waiver.