White should now start to defend his pawn center. He cannot play Nc3 because of b4 but he can play 3. Nf3 or 3. Bd3. As little theory is available on this position, it's hard to tell whether there are other good moves. 3. g3 sounds interesting as well as 3. f4 or 3. c4.

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'1.e4 a6 2.d4 b5'

Wikipedia has related information at St. George Defence.