If the picture was taken when he was young, he could have lost the mane when he shed his baby coat, and if so I'm not sure if it would grow back in or not.
If it was a picture from when he was older and had his adult fur, and he lost the mane from the mites and scratching, then it should start growing back in after his next molt.
I've read that some breeders will increase protein in the rabbits diet to help with fur growth. I know some will add calf manna into their rabbits feedings because it's high in protein. However it's not something I would use because of the corn and high amount of starches in the feed. Leafy alfalfa hay is high in protein, but it is also high in calcium so could potentially cause urinary or kidney problems due to the high calcium. These would be the two options to increase protein levels, though they each could have risks with their use.
If you don't need his coat to grow back in because you plan on showing him soon, I would just let it grow back in naturally after his next molt.