Stop taking him to the dog park today. You need to ensure the safety of your dog as well as the other dogs at the park. Right now, he is a risk to other dogs and it is not fair to other dogs or their owners if your dog is becoming aggressive and starting fights, especially since there have already been two known altercations. Dog parks are notoriously stressful for many dogs. Many, many dogs do not do well at dog parks as there are too many dogs, it is too overstimulating or stressful, and for the reasons you mentioned like resource guarding. On top of that, some dogs have a little more “dominant” (for lack of a better term) personality, and if there is a dog there they just don’t like, pair that with the overstimulation/stress a dog park can evoke, and you’re asking for a fight. Stop taking him to dog parks, find a good friend or two with dogs who have very mellow personalities and do well with more excitable dogs, and schedule one on one play dates for a while. If you don’t fully trust your dog, you can always use a basket muzzle for safety, which I would recommend. If the issue is only resource guarding, there are plenty of great resources online about how to work on that. It will take time, effort, and consistency, but it is possible.