It's my belief no dog is born wired wrong. But we often don't understand how to relate to dogs, that is exactly how they ARE wired-up, and at the first chance they will try to make a bid for leadership. The more this happens, the more the habits get entrenched, yet even a short time of doing exactly the right things can re-train any dog.
Now this is a subject unto itself, and I am no expert on dog handling and dog behaviour. (I am extremely fortunate to have a dog who must have come from the Angels! lol! And have no hands-on experience with this kind of thing.)
I think you need a good dog-behaviourist to work with you and your Boxer.
Things can't go on the way they are. Sooner or later you will get injured, or possibly your dog will pull free and cause someone damage or even maybe get killed on the road!
You need a professional.
You could ask at your veterinarian's office about this. Or maybe if you do a little research you might find someone in your area?
Hi, Take your dog to the vet for a check up. Did your dog go to puppy school? Have you tried a harness it helps to control the dog or a halti that goes over his nose.? Have you looked at the website caesars pack. That has halped me a lot in how we control our suoer fit large dog. A choker chain right up at the back of the head does not like nice but is effective in ciontrolling him. When i don't feel like doing it for the day as it is exhausting i throw the ball until he is tired and let him in for the rest of the day with us. I also throw the ball for a while before i walk him to wear off some of that excess energy. Good luck hope some of this helps. Cheers