If you see clear edges start to grow it's healing. But honestly since his rays were taken off too its very unlikely it will ever grow back. I don't think rays grow back
But he may be able to regrow a little and it will be easier to swim.
What is the current like in your tank? I've found that with my biters and fin rot cases that putting them into an unfiltered tank with absolutely no flow promotes better and faster regrowth. I believe its because the delicate new tissue is able to get stronger without being battered by the fish swimming against currents. But that's just based on my personal experiences :)