I love Special Effects for bright and long-lasting colors. Manic Panic has good colors but, IMO, they wash out too quickly. All dyes will be MUCH brighter if you bleach your hair out first, but ... kid. Reds shine the brightest and fade the fastest, I think, but as usual YMMV.

Also, rinse his hair with white vinegar when washing it out, and have him go easy on the shampoo for a while.

Some people swear by dyeing with kool-aid (the kind with no sugar in it). It works, but it's very faint and temporary.