Blood in bulldog's stool?

I have a 7 month old Bulldog, I got him when he was 4 month old and ever since we got him he has had bloody stool. Now he has been to the vet and checked more then 3 times and he comes up negative for any type of diesease or parasite. Weve changed this dog's food type more then 3 times as well. Could this possibly be an ulcer or maybe the food is to harsh? By the way hes on Royal Canin puppy 32 food.