The spiralling, is exactly what I think... Something internally may have quit or is quitting. BUT, since he is about 4-5 (never know how old they are in the store) he is definitely an old man!
Some old fish, will go out peacefully.. Shiloh did. He slept at the top of the water more, and I knew he would pass away from age soon... And I was right. Others, don't like to go out without either a fight, or something going wrong. Example: For people, some elderly people pass away peacefully while others get terribly sick.
He could pass away soon, just so you do not get shocked when he dies. But it's from age! :) Keep him as comfortable as possible, still offer him his food, and keep up on the water changes... Try not to remove him from the tank. Instead, use a turkey baster to get the gunk from the bottom... And use a cup to gently remove water, and re-add water. That'll lower stress.