It is not likely they would approve another move. Was he given any choice about this? You can contact the facility you want him to go to, and they can contact the insurance to see if they would provide the required pre-approval.

Otherwise, you are going to have to push HARD to get him the therapy he needs, identify achievable goals, and prepare to get him home (NOT to a nursing home) once he is done at Health South. If he stops making functional gains, he is likely to be discharged again pretty quickly, since his insurance will undoubtably be watching closely. Any failure to show gains from week to week in FIM scores will probably be enough for them to stop paying.