Although Rainbow Boy has yet to make a proper appearance, he did appear in a one-panel cameo in PROJECT SUPERPOWERS #0, as one of the victims of The Fighting Yank and his urn. (Although he looked a bit more like Rainbow Grandpa...)
It's kind of interesting to note: that while The Flame and The Black Terror are constantly worrying (and whining) about their sidekicks, Hydroman (who teamed-up a few times with Rainbow Boy, including RB's first appearance) doesn't seem too concerned about his friend.
Rainbow Boy first appeared in HEROIC COMICS #14 (Sept 1942) and stayed there through issue #20 (Sept 1943). He made one final appearance, in a team-up with Hydroman, in HEROIC COMICS #25 (July 1944). And then he was gone...
This Rainbow Boy story concludes yesterday's Hydroman tale from HEROIC COMICS #19 (July 1943):