Akitas and German Shepherds are both big powerful breeds, and both can be reactive to other dogs. I have no idea what Akitas are like as puppies, but with the behavior you are describing, I think that while it is not caused by the dog park, it can be making it a lot more difficult for you to manage him away from that setting.
Your puppy wants to run and play with other dogs, but the leash is giving him mixed signals. He may feel like he has to guard what he is connected to. He may feel some anxiety because he is smart enough that he knows he cannot run if a dog does challenge him. Usually he has two choices, fight or flight. And, when they know that flight is no longer an option, the only thing left is fight, so they tend to react before they figure out whether or not the other dog is a threat.
Also, he is feeling your anxiety -- how is he going to act, will I be able to hold him, is he going to try and attack those other dogs, what will they think about him, why is he making so much noise, and is this going to get even worse? This is all being transmitted down the lead. And when you tighten the lead and hold him in a death grip, he feels that much more anxious.
At this point your dog isn't a candidate for puppy classes. If he needs to go to any classes, he would be ready for basic obedience or basic household manners. Puppy classes are for dogs that are between 10 weeks and 5 months -- usually starting before the dog dog is five months old. Of course other puppy owners are concerned when a dog that looks grown and is big and is acting like a nut is in the class with their puppies, because a bad experience your dog can be really bad for their puppies.
On the other hand, I agree that your dog is probably just a large, over-grown puppy, that needs boot camp.
I think that your puppy will improve if you do a few things:
Management: If you are having trouble controlling his size, then muzzle him in public and use a prong collar so that he does not drag you into other dogs.
Leadership: Look up NILIF (Nothing In Life Is Free). Read through it, and change your leadership style so that he is VERY clear that you are a strong leader and can protect him, give him directions, and that he can look to you when he is unsure.
Exercise: Right now he is full of energy. Before training get him out and throw that ball for 20-25 minutes, then go for a walk. Several times a day tire his butt out. Then do five minutes of training. Exercise his mind and body. A tired puppy is a good puppy. If you need a muzzle and prong collar to walk the dog, it is far better for him to get the exercise. Use a basket muzzle if necessary so he can pant, etc.
Training: You need to do regular training with this dog all the time. Lots of praise, lots of treats. Everything he gets, he should do something for it. Make him SIT before opening the door to go out. Make him lie down before you give him his bowl of food. Train him to WATCH you and give him treats for doing so. Train him at home with no distractions. Get good at the basics. Build the bond. Add in distractions slowly. Start with walks in low traffic areas. Then increase it. Start with the distance between what he reacts to as pretty far away, then reduce it. Try to stay under his threshold if at all possible. Call in a professional trainer to work with you and him alone, and possibly with a dog that he brings. Then after he is showing good progress, a group class would be beneficial.
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