Complex sentence: There is no evidence that he was the inspiration for the song; however, since it was written before the U.S. entered the war he may have been dubbed the Bugle Boy from Company B in reference to the song, not the other way around.
Split into simpler sentences: There is no evidence that he was the inspiration for the song, however, since it was written before the U.S. entered the war. He may have been dubbed the Bugle Boy from Company B in reference to the song, not the other way around.