Welcome to YT.
How old is he?
If he isn't a puppy anymore and his teeth are discolored, it is very likely that he needs a dental and the vet needs to look around in there and see what is going on. My Yorkie needed nine extractions at age five and if it would have been let go, the chance of organ dysfunction (heart, liver, kidneys) from the bacteria would have been very high. In a healthy dog, anesthesia is safer than letting their teeth go. Yorkies are notorious for this. His teeth will need to be brushed at least 3-4 times a week. Did he lose all of his baby teeth?