Is there a good diet I can give my 6yo if he has polycythemia?

Yes. It depends on the underlyihng cause of the polycythemia. E.G. Living at a high altitude. I have seen it as a result of thiamine (vitamin b1) deficiency affecting oxygen censors in the brain. Everyone requires "god made" food, irrespective of any disease entity. Don't just "except" the diaghnosis blindly.