Sorry to hear Charlie is going through a similar thing :/
I’d have them do a preventative mites treatment just in case. It’s almost impossible to see them yourself unless there’s a severe infestation. Does he have bald patches? You could have the vet do a skin culture and check for a skin infection (bacteria or fungal).
I took Yuki to the vet on Sunday mainly because of his vomitting but I’m also concerned about his quill loss as it’s been going on several months and I’m worried it will continue to increase like Crazyflyinaces hedgehog has now experienced. The vet said Yuki’s skin looked healthy and he wasn’t too concerned but I was(!) so he gave him a topical revolution treatment (do not let them use Ivermectin! It can be easily overdosed if injected and can be fatal on the second dose). I have only found one quill since so that’s an improvement already.
Other things to consider is his diet, what brands do you feed?
Also what do you use to bath him with (what kind of soap, shampoo or a wash?) and how often do you give baths? How many quills is he losing per a week and when did it start. It’s good for us to all compare and see any patterns. It could be something simple likes mites, husbandry, diet to environmental issues or an underlying internal health issue.
Hope he improves soon, let us know how it goes at the vets too.