10 week old English Bulldog

Hello, I am new to this forum and am hoping to find answers. We recently brought home our new puppy. The breeder was feeding Blue Buffalo fish and oatmeal “adult breed” with royal can in puppy mousse. After coming home we stuck with the exact same regimen and changed nothing. Our puppy developed horrible diarrhea for about 3 weeks. After putting him on a prescription diet, vet recommended along with a digestive supplement, cooking for him and adding pumpkin, it seems that his gut has normalized. It is now time to slowly get him off of the prescription diet however I am still overwhelmed by the choices here. I am looking for a dry kibble in the medium price range. I went into this thinking Blue Buffalo was a decent brand but after reading this website it doesn’t seem to be the best choice. Any suggestions on where to start my bulldog. Being a breed prone to health issues, skin irritations, joint issues, etc,, I would like to get a jump on feeding him properly now before things get out of control. I am looking for any and all suggestions on food, suggested supplements, additives, routines, etc… I also am not clear as to why the breeder feeds his puppies adult food instead of puppy food, any thoughts on this. Also, are english bulldogs considered large breed or medium breed on this website? Thank you for any and all help.