I would have to say the spot are just a change in the skins pigmentation..My pekingese has alot of dark spots on her and the older she gets the more she gets of them.My little boy Peke has little spots all over his body and he gets more in the summer and like Ginger said about her baby his goes away after Summer.
If the spots aren't raised, scab-like, inflamed in any way, but instead are just like freckles, then I don't think that's anything to worry about. My previous dog, who was a white dog, developed these freckles both on his testicles, and on his belly. And it wasn't age-related for he had them when he was younger too. It WAS sun-related. He always developed them in the summer, and they faded away in the winter.
Also, they were not related to skin cancers of any kind. He never developed skin cancer.
Just like people, dogs can definitely have changes to their skin pigment as they age. If Your vet has seen him I would trust Your vet, a vet is able to tell a normal skin discolouration.
i think its age spots, i may be wrong but my nans dog had them all over his belly