I know a dog who has something similar. The Vet thought she might have an infected tooth. But it turned out she didnt.
She was not a dog who could have her mouth easily examined by a vet. The Vet tried two different antibiotics, the second time it killed it. He was going to put her 'under' if that didnt work.
She has been fine since last summer.
That is all the experience i have to offer.
It does sound like it could possibly be a tooth infection/abscess. If it gets really painful your dog will probably stop eating. But often dogs with tooth problems keep on eating anyway, unless it's really bad. I'd eliminate that possibility first.
Is it easy for you to examine your dog's mouth? Sounds like it would be on the bottom jaw. You might see something. Or get the vet to check him out. If it is this, antibiotics will likely sort it.