Tuck is having yeast issues

....I think it's yeast, anyways.

About a month he had some problems with one of his ears, which is not really unusual for him, but this was pretty bad. We did a lot of ear cleaning for a couple of weeks and it was better. It was enough though, to convince me to change his food.

He was eating Pro plan sensitive skin and stomach. He's always been a kinda itchy dog, and sheds a lot, and had some redness between his toes, but nothing major.

We spent ages in the store reading labels and decided on Natural Balance Limited ingredient Bison and sweet potato. Somewhere around this time he got a hot spot that required a vet visit.

It's now a month later, and he's got scabby bits on his lips (from scratching) and it looks like the fur around his mouth is falling out. He's also been licking his feet a lot. He's defintely worse.

In retrospect, if it is yeast then maybe the sweet potato was a bad idea, but where should we go from here?