I'm so very sorry Jeanie to read that poor Tibbe is suffering
If you decide you want to try and change his food then a grain free diet can sometimes help with yeast issues, YT members may be able to list some safe food brands for you to try since I'm not familiar with all the US brands. Hoping other members might have some better suggestions for you.
Hoping Tibbe finds some relief soon. Hugs to both you and Tibbe from myself and Pebbles.