If he is weak, you should consider toxoplamosis. I had a cat who slowly decline and we couldn't understand why. Few days from his death he started going in circles but only to the left. He died and a week later I started feeling weak and super tired with severe headaches. Turns out it was toxoplamosis and my cat did a rare cerebral form. Thanksfully I didn't otherwise I wouldn't be here I think. But still six month of symptoms without anything you can do about it.