You might want to read about megaesophagus. I don't know that the symptoms are exactly as you describe, but I do know that sometimes it takes a while to diagnose this. I suppose if he is able to digest some of his food and he is not losing weight, this is offbase....but might be interesting for people to read about.
I don't see this condition mentioned much on YT and it is actually fairly common.
Canine Megaesophagus, Aspiration Pneumonia & Myasthenia Gravis