You need to take him to a vet ASAP. Without a pathology report, no one could say just what it is. Also, almost everyone here is pre-vet. We do not have degrees, and even a licensed vet could not make a diagnosis over the net. Certain breeds can get small benign tumors throughout their body. This 'lump' could be anything. Your dog could be getting weak because of a different problem having nothing to do with the lump. Please take your dog to be checked out, especialy if he is getting weaker by the day.
DrRae