I think they said switched him TO Purina...which IS sorry food. Wheat Corn and Soy should always be avoided. Chicken can also be problematic.
Try a grain free diet with a single protein. Duck formulas have never let us down.
For a yeasty ear try flooding every other day with 2-3 parts water to 1 part Apple Cider Vinegar. Flood the ear canal and massage it in. Then wipe up excess with paper towels and cotton balls.
Like Chip said, Purina is not good for dog but great for cows. Since there’s allergies, get single protein bags and rotate after each bag to find out what he can’t have. Stick to Acana, Zignature, Fromm.....For the ears Chip gave you a good recipe. Or.... I use Lavender Essential Oil. Couple drops ABOVE the ear canal as oil seeps on its own into the ear. The brown guk will disappear within a couple days n I use it as maintenance. Every other day. Use a quality oil like DoTerra, Young Living, Woolzie...