died of cancer
on December 3, 1999. He was 57.
He was originally a jazz singer and pianist, but gained a tiny amount of fame with his hit single "Scatman" in the mid-90s. By this time in his career, John was scatting over techno. Scatman John's achievements are rather amazing, when you take into account his terrible stuttering problem. He was able to temporarily overcome his speech impediment by focusing on his singing and scatting.
More generically, a scatman is any singer of scat, a type of jazz based on rapid vocal sounds and nonsensical singing. It's a real genre and a rather wonderful talent, not something freakish.
John may have been a one-hit wonder, but at least his one hit was something very unique.