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At 13years of age, you've been doing very well taking care of him.
I would keep him on his normal diet (the normal routine that you go by) and just give him some extra vitamins to see if that's help.
If you did get the Poultry Cell, you can use that. Give him a direct oral dose of 1cc per 3lbs of weight once daily every couple of days.
If you haven't gotten the Poultry Cell, then I'd go to Walmart, CVS, etc. and get a bottle of Vitamin E and B-Complex. Give him 400IU Vitamin E and 1/2tablet B-Complex daily for a couple of weeks. Give him some egg to help with the uptake of E.
Have you looked at his legs/feet to make sure that there's no scabs on the bottom of the feet or toes?