Hmmm, I dunno. If it's only on one paw and he's limping, to me that would sound more like an injury. I would highly encourage you to have a vet make that call. Our Bullies may seem like big scaredy cats sometimes, but they have a tolerance for pain like a brick wall and if they're limping for any extended amount of time, I would be worried.
*Edit; Although, if you just recently switched his diet over to a chicken based formula, do keep an eye out for a while for other allergy related symptons. Not every dog is the same, but some have been shown to be allergic to chicken.