I think he mentioned in an interview ( don't remember which,though ) that he learned just the songs he needed for August Rush, but that he didn't actually knew hoy to play guitar or read music.

Far before he got the part on August Rush, he had known how to play music. But he plays clarinet, not guitar. And then he starts to play guitar for 18 months for August Rush. But so it means, far before August Rush he has known how to play music because he has played carinet for a long time.