I'm just an amateur on guitar too. It was difficult for me to figure out his tuning at first. He actually tunes down a half step similar to how Jimi Hendrix, Stevie Ray Vaughn and other guitar greats do. I think it makes it easier for him to sing along. I'm not sure if that will help you on the harmonica or not, but once I realized that he was tuned down, everything came together for me.
I've found reggae to actually be similar to 12-Bar blues in some songs. For example, Stir It Up uses the same I, IV, V (A, D, E) pattern that is in blues music. Hope that helps.