Sudden behaviour changes can happen when some physical thing is happening, but it is "pre-clinical" (that is there are no symptoms of anything showing.)
When you say he is in excellent health physically....do you have evidence of that? Has he had a full blood panel run in the last 6 months? And a thorough check by a good and trusted vet?
No doubt he IS in great health and he's just going through a "sentimental" phase! lol.
But if you feel like doing so, you could take him in for a thorough check-up -just to be 100% sure. Get his blood-work done. It will cost a little, but at least it should set your mind at rest as to any possible hidden issues. And the good thing about that is, if there are any, you will catch them at very early-stage.
Otherwise...he wants to stick very close to you....so there are a hundred reasons psychologically why he might do that. One is separation anxiety. Does he get that? How is he if you have to go out and leave him alone?
Another is, could he be sensing that YOU have worries or problems right now? Or your mind is miles away for some reason? Dogs (and cats sometimes) are extremely empathic. They sometimes sense things that are bothering us before even we do!
Another physical consideration....how is his hearing? And sight? If by any chance he has some weakness there, it makes sense he would want to stick close to you if one of his senses isn't working 100%. That can also be tested by a vet.