Not sure how old this post is but our boxer had the same thing. The technical term is Season Flank Alopecia (?). It happens most in the fall when the days are shorter, it is a lack of sunlight. We were told to give her Melatonin (3mg twice a day) it cleared right up and now we giver her one a day to keep it gone. It doesn't have any side effects, even though it is a human sleeping aid. Good luck!