Awww, what a beautiful young lab. I fully understand your postman, as my labrador (a yellow one) only died last November, at the age of 15. What would I not give to have those puppy days again, even though they were also hard work!?
I am glad you got back to the park. I hope you let little Mr. Labrador off the lead to have a blast with the rest of the dogs. It is great for their exercise and socialisation levels at that age.
Young labradors are very energetic and bouncy. Training them takes time and isn't the simple matter that so many people think it is. If anyone ever tries to tell you that theirs has never upstaged them then I would be inclined to take it with a large pinch of salt.
I once encountered a lady who claimed to me that her spaniel had 100% recall and would never run up to another dog or go into a ditch and fail to come out at the first recall. The dogs had given each other a sniff and play-bow each on the way past, nothing else, but she took offence.
I walked in the same field the next day. Lo and behold, there she was again. This time shouting and shouting at her dog, which was playing in the ditch having a rare old time and absolutely refusing to come out. 
I didn't laugh!! Of course not!! She noticed me and was very red in the face, looked at me but didn't speak this time.
Your dog is lovely. You are doing well. I am now nostalgic for having a labrador, and I miss my big yellow boy so very much. I will hopefully have one again one day, and will be far more prepared for how tricky they can be to train and how long it takes. Especially at the age yours is, which is really still the doggy teenage years. He was a right "Kevin the teenager" when he was that age. By the time he turned two years old he was almost out of the other end and beginning to listen to me again.