I agree with talking with the Vet and see what they suggest. With his age, you don't want to take any chances. It is so hard seeing our furkids age.
Does he whine when he gets up and down? Does he seem to be in pain? Is anyone in your household sick? Has he been around anyone who is sick? I think that they sometimes pick up viruses like we do. I know there are times when I just feel like blah and want to lay in bed all day. Keep us posted. I'll be thinking of you.