Any guesses on what my doggo might have??? Skin issues.

My schnauzer is 10 y/o and he’s starting to have these rough patches on his skin. He started biting down on one of them recently non stop. The area is now red with no fur. He has some black spots all over his skin but those have been appearing as he’s gotten older. I’m more concerned about the sores and hair loss I see in the picture. He’s eating some low fat GI food the vet prescribed about a year ago and has had no problems since. I’m taking him to the vet next week, but any guesses on what he might have?