His back looks a bit bald in places in my opinion. My boy's back can look a bit sparse when he's balled up sometimes but that's because he's gotten a bit big and so the skin is stretched. This looks more balding and larger areas. Also when he isn't balled up I can't see any bald patches. However when I first got him he had bald patches (smaller ones) and lost a few quills. It's a good sign if you can see new quills growing in though.
What bedding do you use? Also are you feeding a good quality kibble? I really noticed a difference with my boy's quills/patches once I changed over his diet to good quality kibbles. Let him settle for a little while before changing up his food if he isnt already on good food though. Also once he's settled it might be worth taking him to a vet for a wellness check and they can check for mites at the same time.