Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned about Shep, and rightly so because things are changing for him.
This isn't bloat (gastric dilatation and possible volvulus, or twisting of the stomach) however. Dogs with bloat are attempting to vomit but unable to do so, have a belly that looks distended and is rapidly becoming more so, not slowly, won't lay down and settle may stagger around and paw at their mouth and be unable to drink water normally.
Your fellow is eating and drinking just fine, and isn't retching unproductively.
So what may be going on?
If he is losing muscle strength then abdominal muscles may be affected too, and that can lead to a pot-bellied type appearance as abdominal contents aren't held in place.
It's also possible that given his great appetite and muscle weakness he may be developing Cushing's disease (hyperadrenocorticism). This is generally due to a benign pituitary tumor which increases adrenal gland stimulation and cortisol production which in turn causes weak muscles, increased appetite and water consumption and secondary organ enlargement, especially of the liver (which is located in the abdomen just where you are noticing changes).
The other possibility is that he has an abdominal tumor. These won't necessarily make him feel bad until they get big enough to interfere with passage of food or organ function.
I don't think your fellow needs an immediate veterinary visit or emergency care this weekend, but I do think a trip to his veterinarian for an examination and some abdominal radiographs would be wise fairly soon.
Please let me know if you have any further questions.