My 8 year old boxer will not eat 3 to 4 days a week in the morning. You can hear noise from his insides from 15 feet away. I best can describe it as he has a cat in his stomach (which he does not). His diet is the same as always, a mixture of dog and human food.

You could have a vet check him, but I would dump the human food. Any decent, modern dog chow is carefully formulated to provide the right balance of everything a dog needs. It is beyond most dog owners to do anything except mess it up by adding this or that. There certainly is no good reason too. I have personally seen thousands of dogs thriving on Pro Plan and little else. I know of thousands more doing equally well on other brands. Unless it develops allergies, it is rare for a dog to have problems eating a dry dog chow. Even with the big recall panic, none of the major brands of dry kibble were recalled. If a dog develops an allergy to chicken, it doesn't matter if it comes out of a Pro Plan bag, a Chicken Soup for Dog Lovers' Soul bag, or the grocer's freezer, it's chicken. Likely a simple switch to a lamb and rice chow will fix the problem.

You may want to phase out the human food. Any sudden change in a dog's diet can cause digestive upsets. After he is on all dog chow for a couple of weeks, see how he does. If he refused to eat the dog food only, chances are you have been over feeding him. See https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171

Did this just start recently? If so he may need a vet checkup. If it has been an on going problem, Then maybe you are feeding him too much and he is just not hungry the next day. I had to adjust the amount of food I was giving my dogs. I was over feeding them and throwing perfectly good food away at the end of the day.