A great talent who moved easily between the worlds of gospel, R&B, and Pop music. He was a musician's musician, the kind others love playing with. Billy was probably best known for playing keyboards on several late Beatles tracks (see him playing on "Get Back" here), but by then, although still young, he was a show-business veteran. Billy was performing on TV shows by the age of 11 (see him here with Nat King Cole, and in his later teens here with Ray Charles).
Here is Billy playing one of his own songs, "That's the Way God Planned It" at George Harrison's Concert for Bangladesh in 1971.
And here he is 30 years later singing a heartfelt rendition of "My Sweet Lord" at the Concert for George.