The dog park is no place for a 15 week old pup. He should not be in that environment until he has all of his shots for the year, and he has been neutered. My boxer did not enter the dog park for the first time until he was 6 months old. You gotta remember that other dog owners are not always responsible. Why chance your pup getting something bad because he is not fully vacinated for the year.
You also mention he will ONLY eat Purina. He is only 15 weeks old, there is no "He will ONLY". You should change to a better food if its within your budget. He will eat it if he is hungry, and the sooner you get him used to it, the better for his long term health. A dog will not starve himself. If you are free feeding him, try doing 2 or 3 meal times a day instead. If he is hungry, he will eat the food when its placed down.
Welcome to the world of boxers. Please realize that people here give advice to help other members out, not to tell you everything you do is wrong. You seem to get a bit defensive at advice offered. None of us know it all, so advice from others always helps.