I would suggest taking the dog to the vet and have some tests done. See if thyroids are normal. Sometimes with abnormal thyroids, medications can help ease anxiety and help ward off aggresssion. By no means would it cure aggression, but possibly make it easier for you to handle and work through.
You did everything correct, meeting on neutral grounds and handing out treats, going on a walk. It just sounds like your dog has had zero socialization
Even in a new area with a new puppy you could have taken him to pet stores at the very least. People at pet stores love dogs! I have had no problem taking my rottweiler to petsmart and petco and getting people to pet him. Out on the streets is a different story, but at pet stores you find dog lovers, which is exactly what this breed needs. They need to learn from a young age to accept friendly people.
Can anything be done now? Yes. Will it be easy? No, because his critical socialization period is over. That is why you need hands on professional help. If you want to help this dog, spend "an arm and a leg" and help him!