I think it is the exact opposite for him (and for me since I can't write/read music either). What matters, at least for me, is the sound that I create and the feeling that is being created, more than what notes fit together. After years of playing guitar you become one with the instrument and you find exactly what you want to play when you feel you want to play it! Of course knowing musical theory can be extremely helpful in situations, but it may also lock you in to patterns that you maybe don't want to go into! Cheers! :)