If it's for a bluesy sound, the harp is usually a fourth above the played key. So, he plays a song in G, the harp needed will be a C harp. However, since he tunes down a semitone, the harp needs to go the same way. It needs to be in B. When he uses the open G tuned guitar, the far more common C harp is the choice.
If it's a German made harmonica, you'll be looking not for 'B' - 'cos that's Bb in English, but H!