I would switch him back I think. He could be having an allergic reaction to something in the food.... Compare the ingredients and see what the differences are. Switch him back and see if he gets better, otherwise you may want to make an appointment with your vet if it doesn't. If it does goaway, try the food again in a couple of weeks and see if you get the same reaction. Then you can narrow down a possible allergy.
I cannot believe that there is not a good forum on at bernards out there! . Dont worry about asking a non EB question here either- we love all pets!