If you want a dog who walks politely on a loose leash you must teach him that this is what you want. Get out some really yummy treats. Put the dog on the leash (starting in the house), hold your hand containing a treat at your waist, and begin to walk. He will be interested in the treat so he will stick by you. Walk around and every once in a while praise him and give him a treat. Then move your practice sessions outside. Practice having him walk by your side in non-distracting environments before going to the park. Start in the backyard and then move to the sidewalk in front of the house. Build the behavior through praise and treats. He will want to stay by your side if you are the most interesting thing in the picture.
Another method is to “become a tree” when your dog pulls you. Start walking, and whenever the leash becomes tight, you simply stop, plant yourself like a tree, and don’t say a word. Your dog will eventually look back at you to say, “hey, why aren’t we moving?”. When he does this he will most likely move slightly toward you, loosening the leash. When there is slack in the leash, start walking again. He will eventually learn that when he feels tension on the leash, he doesn’t go anywhere, but when the leash is slack he is allowed to walk.