He shouldn’t have a deficiency being on a balanced feed, but I would try some B complex. It can’t be overdosed, and might help. If it helps, it should help within a couple of weeks. I just worry that this has been going on so long, it may not be curable. Mareks disease can sometimes cause...
He needs B complex which has riboflavin, and all of the B vitamins. Give 1/2 tablet daily. He could have riboflavin deficiency. Riboflavin deficiency can become permanent if not treated right away. What do you feed him?