I would discuss it with a pediatrician, but if it doesn't burn your child's bottom and doesn't smell unusually bad then I wouldn't be too worried. I'd still check up on it with a pediatrician but I don't think it's cause for major concern.

Anything with food coloring in it could be causing this. The "Froot Loops" for breakfast or chewy fruit snacks can do that.

Either that he is allergic to something. But if he seems in good health, it's probably just the food coloring and not something to worry about.