My dogs’ breath has always gotten a bit that way as they have gotten older. not quite so bad as you describe, but somewhat. With the older ones I have had some (limited) success with minty stuff (it settles their stomachs a bit as well as the obvious mint scent thing.) Now there a a bunch of mint chewy things on the market that there weren’t around with my last old dog, maybe trying them would help? Ask your vet about trying a homemade diet, I’ve heard that can make a difference, as well. Gotta love those smelly old boys!