Are you doing your own mucking and providing bedding? If you were on pasture board previously, and 1.) are now using a stall 2.) therefore using more hay and more bedding and 3.) the BO is doing any of the work related to having him in the stall, yes, it's reasonable to the BO to charge you more while the horse is on stall rest.
Would have been nice if he handled it or addressed it in a more professional manner, but yes, it is reasonable for him to expect more money. Without more details on costs in your area, I don't know if $200. is reasonable or not.