u/Informal-Lab484

Hi everyone. I’m hoping to get some ideas before our vet visit because I’m really worried.
My dog used to have a healthy coat, but recently he developed severe hair loss, redness, and sores like you can see in the photo. He’s never looked like this before.
Has anyone experienced something similar? If so:
What ended up being the cause?
What tests did your vet run?
Was there anything that helped?
I’m not looking for a diagnosis, just hoping to hear from anyone who’s gone through something similar so I know what questions to ask my vet.
NOTE: This is months old picture. He doesn’t look like this now but there are few red marks as the medications did help but not fully.