He's got dark hair, right? If he really wants to go blue, and doesn't mind semi-permanent (at least untill it grows out enough to cut off) I'd recommend something like Manic Panic. However, the color will most likely be subtle unless he bleaches it out first. If he's just looking form something temporary, they also have Dye Hard styling gel. Hot Topic carries some wild color hair products too. Of course, he's going to have to wait until the food coloring washes out. I recommend frequent washings and lots of soap.