If it helps any, the earliest reference I could find of Supes described as a boy scout was on page 25 of the essay The Spawn of M.C. Gaines, written by Ted White, the opening piece in the nostalgic essay collection All in Color for a Dime. All was published in 1970, though Wikipedia says that the pieces contained therein were written in the early sixties.
I should have included the context of the quote and the quote itself above: White was discussing the unlikelihood of a Golden Age young Superman fan caring whether the creators (Ellsworth, et al.) got the physics of Superman right; in the next paragraph, he said, "Superman was a myth-figure: he was our dreams personified, even as he must have been Siegel and Shuster's. Superman was, almost literally, the perfect Boy Scout. We still believed in Boy Scouts then."