If this really did come on quickly and you didn't miss the onset, then being as you're from Michigan I would have him tested for Lyme disease. When the bacteria get into the pericardium this is what you see first. Especially when it's accompanied by Lyme caused kidney issues. If he shows gait issues I would have the chest Xrayed and look for heart enlargement. A Lyme titer may be not be diagnostic since if he has been vaccinated it will show up positive
What happens is the bacteria get into spaces like joints and the pericardial sack. The immune system recognizes them and the macrophages go to work gobbling them up. The lysing enzymes also break down tissue which overloads the kidneys. If you stop it early enough with antibiotics and judicious use of a steroid you can modify diet to keep the dog going well for some years. If this is what it is, he'll likely never hunt well again.
