Ummmm Use crayon…….?

try some plain water. if that doesn’t work then try some soapy water with a mild soap in it like dial or ivory dish soap. just be sure to test a spot he won’t see so that if either of these suggestiongs are going to stain anyways he won’t have a really big one in one spot. good luck.

try that tide pen for stains on clothing.

Recolor it with one of those Sharpie pens.. hey, it works in the commercials!

There isn’t much you can probably do since what you used probably bleached the color out. You will need to to find something to recolor it like Rit dye. (or a Sharpie)