You did not mention how long the raised bumps have been on your puppy. If they have been there since birth it could just be that he has a very prominent spine. If they just occurred, they could have an allergic cause as mimi has mentioned in her comment. Hives are common in dogs as an allergic response.
Has your dog come in contact with something that he may be allergic to? My puppy had to be rushed to the ER after he got a vaccine (he is allergic to vaccines) and that was one of the symptoms I noticed- hives and they felt like big bumps up and down his spine. Hope this helps.