Why is my Westie constantly itching his face?

I asked this question before about the itching, and afterward went to the vet. The vet put him on a hypoallergenic dry food, prednisone pills, and gave him a shot for possible mange. (If it is mange he will get more shots.) He does look much better than he did and the prednisone masked the itch, BUT he hasn't taken it for a few days now and the itch is back. He itches his face like crazy. I have given him very very few treats or anything besides the food for the past few weeks. I'm thinking that this isn't a food allergy. I'm really hoping it's not mange, because my other dog will more than likely end up with it.