When I got my lionfish he didn't want to eat anything dead at first either. I put him in a smaller tank (30 gal) by himself. I then fed him the live food he was use to. But only enough to keep him hungry. Then I stopped feeding him live for about 4 days, only offering him the frozen stuff. You might want to try and do this with the grouper. It took a while but it did work, and now he only eats the frozen stuff.
So go ahead and give him the food he is use to eating all the while offering him the frozen. Don't know if this will work but hey it's worth a try
Also if he is by himself in a different tank, it might go after the food better.