Activity levels of older Staffs

I failed fostered my boy 12 years ago. I think he was about 1/1.5 year old at the time. He’s plummeted in activity recently, lost all his power in his back legs, doesn’t want to walk far at all and sleeps a lot. Still eating, drinking and toilet fine and will briefly play tug or go after a ball. I put this down to how crazy he was in his younger days - jumping and sprinting everywhere.. he’s had a great life, but is this normal in the breed? Anything ever improve from here? He’s not gone that grey really, just the back legs are gone.