In my experience most guitar players don’t know how to sight read sheet music. Guitar is very different than a keyboard, it’s laid out two dimensionally versus linearly like a keyboard. Sheet music translates to piano playing easier than guitar playing. Basically all piano players learn to sight read from the start, most guitar players never do. Perhaps that’s why improvisation is considered so important to guitar players. We need to be capable of winging it in a song, playing notes that suit the chords on they fly without being able to rely on sheet music to tell us what notes to finger next.
He was also great with the accordian and violin.
I learned to read sheet music through orchestra all through my school years. For playing guitar, I found it wasn’t worth the effort. The keys were usually selected for compatibility with other instruments. I never liked playing in E flat or B flat but that’s how sheet was scored, usually.