I happened upon this nice video today which talks about his career and his Alzheimer's condition.
The part I didn't realise was this:
In 1963 alone, Campbell performed on nearly 600 cuts for other artists. He played guitar for Frank Sinatra on "Strangers in the Nigh"; for The Monkees on "I'm a Believer"; and for The Beach Boys on many of their recordings. And when Brian Wilson took time off from the band, Campbell filled in for six months.
You can watch the piece here: