Thanks so much for the additional information.
I have several thoughts as to the possible explanation for this change in behavior:
1. Just because a dog has one endocrine problem doesn't mean they can't have two. Cushing's disease is one such possible condition which can cause increased appetite and change in attitude. Usually these dogs are also drinking more water and urinating more and their diabetes is not well controlled.
Sometimes blood work and a urinalysis might be suggestive of this condition (an elevated alkaline phosphatase level and protein in the urine) but usually additional testing such as a Low Dose Dexamethasone Suppression Test needs to be done to rule it out.
This LINK discusses this condition in some detail.
2. He may be suffering from early doggy Alzheimer’s aka Cognitive Dysfunction or senility, unfortunately. This condition is recognized in dogs as they age and is as heart breaking and sad as it is in humans.
The symptoms can vary from one dog to the next but many of them will experience Sundown Syndrome, just like in humans (which means their symptoms only occur in the evenings or worsen at that time). They can start to urinate/defecate in the house, they can start to become confused, many of them pant or whine, they can stare off into space, some of them will "forget" they've been fed,etc.
This can be a slowly progressive condition (in most cases) while other dogs can decline rapidly.
Treatment options are somewhat limited but cognitive supplements such as Neutricks, SAMe (Novifit) and Senilife, or diets such as B/D (which is a prescription diet from your vet) or the Purina senior diet with MCT oil may improve cognition and reduce anxiety if present.
Combinations of antioxidants such as Golden Years (Sogeval) or Cell Advance 440 (VetriScience) may be useful.
High dose fish oils may also help some dogs. Welactin or 3V Capsules are good veterinary brands.
I encourage owners to actively engage and play with their dogs to help with mental stimulation. Regular exercise is also a good thing to keep the blood flowing to the brain.
Over the counter Melatonin may also be useful since it's a potent anti-oxidant and also has sedative qualities for some dogs. The dose would be 3-6 mg once or twice a day. Nature's Bounty is a good brand since quality control issues abound with supplements.
3. When older dogs start to behave in strange ways, I always worry about a brain mass, unfortuantely. But I don't believe his behavior qualifies for something so drastic; it's not unusual enough.
I hope this helps give you and your family some ideas as to possible explanations for his behavior and possible ways to minimize it (especially if #2 above is suspected). Deb