First rule out a thyroid problem, and you should know that if a Golden is in the low end of normal they need medication. Some vets are not aware of that for the breed.
One formula you can use for weight loss is 10 calories per pound you want him to weigh, so if you want him to weigh 80 lbs, he would get 800 calories per day. If his ideal weight is 90 lbs, he would get 900 calories per day. That's for weight loss. If he seems hungry on the reduced calories you can add unsalted green beans to his meals so he feels more full. Once you get him to an ideal weight you would want to adjust his food amount to maintain it.
"To my mind, I hold that the more helpless a creature, the more entitled it is to protection by man from the cruelty of man."
"The greatness of a nation and its moral progress can be judged by the way its animals are treated."
Golden Retriever Rescue of North Texas