Fungus maybe? I'm not real knowledgeable in anything to do with fungus's but I would try finding information about it, see if you can just call your vet with out having to go in and ask some questions?
I read a lot about this recently because I've noticed hair loss on Fidget also. I believe if it is fungus (ringworm, specifically) the skin would appear red or cracked.
If the skin looks simply pink, it could be a massive fur slip? Does he chew his fur?
Also, perhaps his skin is too dry? How often do you give him a dust bath?
There is countless websites dedicated to addressing chinchilla fungus.