Hair Loss on Back

So my handsome boxer is starting to loose patches of hair on his back. Yes we are in the process of switching foods however I first started to notice the hair loss WAY before I even started switching foods. My vet told me to take an oatmeal bath with him for 15 mins and I did about 2 days before I saw them. So I’m wondering what could this be? They also mentioned diaper rash cream! (The first 2 photos he has that on) You can’t see the patches of hair he’s loosing with a blind eye because of his undercoat-but I’m just trying out the diaper rash cream to see if it does anything. If you can help me out please do. If you have any questions just comment and Duke and I will be very appreciative of any help or tips you can give!