I think it would be worth you getting him checked out with your avian vet. Try spraying him with some water and ad some Aloe vera in the water too which will help sooth any irritation. Do also keep him quarantined from any other birds you may have until this condition has settled down or he has been checked just to be on the safe side.
If a bird is lacking in some vitimins it can also cause them to chew/pluck feathers, as well as the stress of your bird being on his own after his friends were sold in the shop
Feather up is really good
Feathering products
Good luck with him, would love so see a photo
hopefully all being well he will settle down soon.