[cypher]

It doesn't sound anything like mites so I don't think you need worry about that.
Is there anything in the cage he could be rubbing against? that can sometimes cause patches like that.
You could try giving a few extra flax seeds which are really good for skin & fur, if that doesn't do it a tiny bit of evening primrose oil can help, only a tiny bit, no more than half a drop a day so I mix it in with oats & just give a little of the mix when they need it.