Why does my dogs breath smell like feces?

Let me start with the fact that he has a vet appointment next Wednesday but I’m trying to figure out why is he smelling that way.
I’ve checked his ears and anal glands. They are fine.
He had a full dental work up last year and his teeth look fine. I will brush his teeth and the smell will leave for a while but then the smell will be back the next day.
His diet hasn’t changed.
He also doesn’t get into feces, so not a problem there.
He is also bathed regularly and I comb him regularly.
He is getting old, so I wonder if that may be it or maybe he has a gum infection.
Even then I don’t understand why a gum infection would smell like his feces.
Oh, yeah, about that. His feces are smellier than normal.
Really smelly.
I’ve noticed dogs seem to get this way as they age. Is it just an age thing?