How old is your dog? What breed is he?
When was the last time he ate and drank?
Is he having any coughing, sneezing, vomiting, or diarrhea?
Can you lift his lip and tell me what color his gums are?
That's good. Yes it is normal to have pigment spots.
Can I talk you into taking his temperature rectally?
He could have heart disease, metabolic disease such as liver, kidney, adrenal gland disease, he could have cancer, he could have an infection.
Since he is older, the list of "bad" things it could be goes up :(
Since he is not coughing or vomiting it is more difficult to say.
If it were my dog, I would take his temp. If his temp is over 102.5 F I would be thinking infection.
If his temp was normal then I would take him to the vet for bloodwork. If the bloodwork was normal then I would get xrays.
Did you take his temperature?
Did he start eating again?