I hope your pup is doing well.
As for eating, I had the worst golden ever when it came to eating. As a puppy up to 6 months old, I thought she was going to eat me out of house and home. Then one day, just like that, it stopped. We did no change to make that happen. For the next 8 1/2 years feeding her was a challenge. I had other dogs so I could not just leave her food down. Sometimes I think she ate the equivalent of 3 cups of food a week. Sometimes she would eat better if we tried a different food, but that would not last long.
My suggestion, have the vet check your pup over. There could be teeth problems and like others have said if he is experiencing male hormones about a female around that might contribute. Have your vet check to make sure there are no physical reasons. He should eat if he is hungry maybe he just is not hungry.
Also do not do free standing food. Give him 10-15 mins to eat. If he does not eat or walks away at the end of 15 min pick up the food. If he is still actively eating leave it. Then the next feed time do it again. Don't keep offering, allow him to get hungry. Also make sure he is not getting into other food that might be filling him up (cat food maybe?)
When my dog was 9 years old, one day just like she stopped eating she started again. She never went really off her food again. Dogs like people can be finicky eaters. Good Luck!