Ah. That is most likely an infection in the leg. He may have bitten by some animal. If its his entire leg I would get him to the vet ASAP. I know this will sound dumb but vey gently and very carefully try and find the top of the bump and gently push on both sides. If a fluid or pus comes out then its an infection. This may pain your dog a little but the infection has to come out. If it gets worse or you can't do it, take him to the vet. Remember I haven't seen your dog so I could be wrong, I can only make pictures in my head.
What Causes Large Red Swollen Spots On Dogs Legs, Im Not Talking About A Rash Either, I Mean Almost His Entire Leg Is Swollen?
Maybe this is FAD, flea allergy dermatitis(sp?) anyway, My dog had it and the vet called them 'hotspots'. The fleas bite and cause all kinds of trouble in that area, and keep returning for the easy pickins, your dog itches n bites, the bacteria (between the itchy and the fleas) makes for a sore area. Hair loss, real red, weepy, almost blistered looking, Yellowish crust on edges- sound familiar? Sulfide. It is available at any good pet store (even walmart-sometimes) Its yellow, and smells bad, but boy what it can do! You have to cotton ball it cupl times a day, and treat for fleas. If its really bad- go to vet for antibiotics. I've always just been told to get the sulfidine... Good luck!