good way to get him to back off. Why, since a puff of air is harmless? Apparently cats associate blowing with hissing—their own sign to the world that a serious threat is near.
If you are close to a hissing cat, you will experience not only the distinctive sound, but also a jet of air being expelled from the cat. So, when you blow air at your cat, you are (so the cat believes) hissing at him, and he will respond accordingly.