My ex-husband wants to sign over his rights, What answer can he give the court so that the judge will agree when they ask why. He has never had anything to do with the child and My new husband wants to adopt him.

He may not have to give a reason if your husband is adopting them. If he needs to, simply say "I believe that all children deserve two parents and allowing this adoption provides my children with something I have been unable to provide them"

But at least here in FL, there is no reason needed.

In fact most likely he will not even have to appear in court. Your attorney files for adoption inclucding paper work that he gives his rights up for the adoption.