Also, it's the standard of care to do a panel of routine blood tests on a 10-year-old dog before any dental work is done.
If this wasn't done for Bently, it's possible he had some underlying disease process going on at the time of his dental work. If that went undiagnosed and now two months have passed, he would be much sicker, and that could explain the symptoms you are seeing.
If Bently did have bloodwork done and it was all okay, I'd be inclined to repeat it now because something has clearly changed.
You mentioned he has really bad breath
. That's usually due to one of the following...
1. A serious dental disease issue (decay, abscess)
2. An oral ulcer or infection
3. A tumor in the mouth
4. A disease inside the body that produces a bad odor to the breath. That means things like liver and kidney disease.
Any one of those things could lead to the symptoms you described of pain after eating, lethargy, and bad breath.
The bot***** *****ne is that if I was Bently's vet and he started showing these symptoms, I'd want to hear about it. It's certainly not expected 60 days after a dental to have problems like this, and it may or may not be related to his dental work, but it needs to be addressed with a trip to the vet.
I hope this information helps.