Expensive answer but I'd say take him to the vets for peace of mind, if not your nose.
If you can't see something amiss - caught food, gums, tartar etc then it might be something else.
One of my mums dogs, recently deceased, had terrible breath but they couldn't do much as anaesthetic was risky for a geriatric. He just needed a damn good scale and polish. A suitable knock out drug was found but they did wish it had been invented earlier.