I put nutritional yeast in their food to add niacin. It should be at the grocery store maybe around the baking or spices. I put one tbs per one cup of food. There’s also vitamin b complex at farm stores that will do much more if you think niacin deficiency is the issue
It looks like his little leg is swollen. I’m not much help but I know some turmeric would be safe and should help a little with the inflammation. Just add it to the food