Robo bald patch - mites? Fungal infection?

[ahennessy1]

Hi there I’m really hoping someone can help.

I took my robo to the vet about a month ago as I noticed he had a bald patch on his back. The vet said that it could either be mites, a fungal infection or skin cancer (but that’s very rare). She treated him for mites and his fur appeared to grow back. I noticed as I was cleaning out his tank this week, he developed another bald patch near to where the bald patch was before. I went to the vets today and upon inspection the bald patch had grown worst and his whole backside is near enough gone. She treated him for mites again and has given me some medicine to give him everyday. I haven’t seen any mites in his bedding and he hasn’t appeared to be scratching himself excessively. What does everyone think it is? I’m so worried. Should i disinfect his cage and remove any wooden items as he has a completely natural set up?

Pics attached...