It's not so much a manners thing. I read online that taking him to a dog park, but staying outside of the fence might help him see how other dogs behave. Also he'd get better used to the alarming amount of smells. I figured this might be even easier if he was hindered from barking, or worst case scenario turning on me. Which, for the record, he hasn't done.
A dog park can contain 50+ dogs... that is a whole lot of over-stimulation for an untrained/unsocialized dog! It would be a much better idea to find a neighbor who has a similar sized dog who is very friendly and passive. Go on walks parallel to them, starting as far apart as you need to be for your dog to feel comfortable and readily take treats from you. Carry a treat bag filled with tasty treats, like chopped up hotdog/chicken/cheese, and a clicker. Every click must be followed with a treat! Click/treat when he looks at the other dog without reacting. Click/treat when he's on your left side (you can lure him to your left side "heel" position with a treat). Be a tree is my favorite method for loose leash walking, as it teaches your dog that he cannot move forward if the leash is taut. Make it a rule of nature to him, that if he feels the leash tense up, you stop, and don't continue until the leash is loose.
Obedience classes are essential for new dog owners!!! Enroll in a local positive obedience course (search
http://www.ccpdt.org/). There, you can socialize him with known friendly, vaccinated dogs under the guidance of a dog trainer. You can get hands-on help with all of his issues. It is well worth the $100-ish to set yourself and your new dog up for a happy life together.
Prong collars and other correction collars should be utilized as a LAST RESORT, when you've completed training and it hasn't helped, or with a dog who is very large and must be controlled for safety. Correction collars can worsen his problems with aggression, by a lot. Imagine if every time he sees another dog/person and pulls toward them, he experiences the pain of a prong correction? He's going to associate people and dogs with pain. That is not good.
