I being a Japanese am ashamed I know little about Yamashita.
But as I recall he was trained by his father who was the President of a guitar "academy" in Nagasaki, and his sister Naoko was also a guitar master playing/recording together with Kazuhito.
As I confirmed today on some JP references, he did not follow any methods to study his guitar (which supposedly made him isolated in more systematically studied guitarist community).
Instead, he grew up playing in a guitar ensemble organized and run by his father, which I think is another key of his style being discussed here, esp. volume or wider dynamics.
And by the age of 12 he was second runner up at Tokyo International playing Chaconne which was, as is always the case, controversial among the jury members saying, the tune is not something a 12 year-old can properly interpret, while the audience was best applauding the prodigy as he was.
When I was growing up Yamashita was always a celebrity, but was of a sort of isolated and lone sample as I got more involved in the Japanese CG commuity where I did not hear or see him. As such, I was not influenced so much from Yamashita, just like the other high-teen guitarists in the commuity back in time. I was not a big fun of him, at least not so much as I was of Shin-ichi Fukuda for example, who was more familiar with and closer to us.