You said you "just finished treatment" for mites .... Did you spray the whole coop down along with nest boxes? Or did you treat individually? What did you used? Have you checked him for mites? Have you seen him eat/drink/poop?
When I get new chicks, I start them off on Med Chick Starter, smallest bag I can get so they have at least a week or two on it. Then they get switched to Flock Raiser for life ... I do put out a separate dish of grit and Dish of Oyster Shells when the pullets reach 18wks or start laying early. I've never had an issue with shells nor laying.
Read about the "energy/protein" laying takes, made sense to me. Saves on storage space and not having to worry with a mixed flock. Did read about how layer has less protein and more calcium .... Calcium not being good for chicks and Roos, although some dispute that. Have read about feeding All Purpose, All Flock and even Grower instead of layer with Oyster Shells offered free feed in a separate dish.