A low heart rate in dogs is usually not caused by diet. Heart rate for a golden retriever should range from 70 to 120 beats per minute. If he is very athletic it could be slower and if you took his heart rate while he was sleeping it may also be slower. More common causes of a slow heart rate are cardiac arrhythmias from heart disease like heartworms or primary heart disease. Hypothyroidism is a very common cause of slow heart rate and metabolism in retrievers. Usually we don't start seeing thyroid disease until 6 to 7 years of age, but I have seen it in a few goldens as early as 2 years old.
A high fat diet can lead to conditions such as pancreatitis which can be life threatening short term and obesity which decrease quality and longevity in the long term. Get him on a good quality diet like Science Diet, Royal Canin, Nutro, or Purina One to name a few.
I would recommend take his heart rate throughout the day at different levels of activity and write them down. If his heart rate is in the normal range most of the time then it is probably normal for him. Also when you listen to his heart or feel his heart with your hand put a hand on his femoral artery and feel the pulse. You can find the femoral artery on the inside of his knee. There should be a pulse for every beat of the heart and if there is not then that is a sign he is having arrhythmias and should be worked-up for heart disease. If his heart rate is staying below 80 most of the time then I would get him worked-up for thyroid or heart disease. Best wishes.
Dog Veterinarian
Over 8 years working in small animal medicine and surgery.