Welcome to BYC. Have you added any new birds in the last couple of months? Was he vaccinated for mareks disease? I would start using poultry vitamins in his water daily just in case of a deficiency. Make sure that he is getting enough water and food, in case he could be kept from it, or can't...
He could have suffered some neurological damage in the attack. I would make sure that he is getting enough water, and you might try putting some poultry vitamins in his water or feed in case of a vitamin deficiency.