Have you thought about taking him to a decent training class? choose carefully, not one that talks about dominance, pack theory or Cesar Milan. Its a good idea to ask if you can visit the class and watch without the dog before enrolling. The class should look well controlled, not too full and the dogs should be enjoying themselves. If chain collars or slip leads are used then walk out, the same if the dogs are jerked around by the lead. The dogs should look relaxed and happy but it might be a bit noisy.
Many dogs do not like to be approached and sniffed at by strangers, especially if they are on the lead. I would be stopping him doing it in case he gets bitten. Don't correct him just keep your distance. If he likes food, then take treats with you on walks to keep his attention. Be sure that you take the treats out of his daily food allowance or he will soon get fat lol. They can be something more special than his dog food, just reduce what goes in his bowl to compensate. I agree with Lankeela, get him settled and not having accidents before you get a pup, also doesn't he deserve all your attention for the time being if he has come from such a terrible place?