Ronny Cox was playing the guitar in the film, but as you note - the soundtrack featured Steve Mandell.   Make no mistake, regardless of who was playing on the soundtrack, Ronny is more than competent on the instrument - he is not just "acting". He is the real deal.

The boy playing the banjo did not know how to play the instrument, and there was actually another boy reaching around to play the chords. Great editing and shot blocking!

Ronny's next "big" acting job was on a television show called Apples Way. The show would always end with Ronny's character playing the guitar.