I'm going with excitability, which you can say is or isn't reactive. My oldest golden will bark at other dogs, at the same time he is dancing because he is so excited. Both my goldens are great at the dog park, they will play but for the most part are focused on me and swimming. There is nothing wrong with using a prong on a golden for training purposes. Have you done obedience classes with him? They are great dogs to train with and love making their owners happy. I will say that a number of goldens I have seen at the dog park did not have great temperaments. Lots of growling over their toys. I was not shocked to see it myself. Like lots of people I used to automatically think goldens = happy go lucky dogs, but that is not always the case. My oldest is the one that will alert all the rest of the dogs and if you didn't know he was a golden and you were at my door you would think he was a Rott. I would start with some classes and work on getting him to focus on you, it shouldn't be hard to do.
Misty- Samoyed Mix, Tannor- Golden Retriever CGC
Robyn- GSD CGC, TC, Midnite-GSD CGC,TC, Brennan-Golden Retriever CGC, Batman-Husky/Greyhound , Apollo-GSD