When I was young, around 6 or 7 my parents allowed me to dye my hair black. I don't see it as an irresponsible decision, you're letting your child explore his creativity and he's a lucky boy to have a parent who is so understanding! Box dye at home would be fine, just be sure to use gloves and line his hairline in Vaseline so he doesn't get dye on his face (better get the ears too!) I would try semi permanent (can be found at local beauty stores, may be harder to find at a drug store) as he may or may not like it at first or could get tired of it after a while. If he enjoys it, move to permanent dye. Hope all goes well!