If none of the current suggestions work, go to a beauty supply store. I used to work in a salon and hairdressers get their hands stained frequently with dyes and hair-color products. See what they have. Keep in mind the chemical elements of the dyes and the dye removers penetrate the skin and enter the blood stream. Make sure you don't put your son at risk to remove the color. You may also need some soothing cream to apply to his hands after the color is removed, like an aloe "after sunburn" gel or the "udder" creams that have healing herbs which can be purchased at WalMart.
be careful. If there is one place where purple hands are appropriate, it's Disney World. Have fun and don't worry about the pictures. These pictures will tell two stories and will one day make you both smile.