You might want to do an expanded thryoid panel. The thyroid levels that come back from general bloodwork, are not specific enough to always point out a thyroid problem.
www.hemopets.com Dr Jean Dodds, does a very good expanded thryoid panel.
I'm not sure how old your dog is, but walking stiffly indicates to me some arthritic changes that can be located anywhere along the leg joints. Do you add cosequin to his food? I'd really consider this, as it truly can be very effective for "general joint health".
Try feeding late at night prior bedtime, and no exercising after his last meal.
I don't have any other ideas, other than thyroid panel. If that comes back normal, maybe seek a consult with a geriartic specialist.
It is a far amount of muscle loss, and given you are keeping up his exercise, I would not expect to see 25% or so loss over 3/4 yrs.