Hm. Well, if any health issue has been ruled out...
Have there been any changes in his environment at all? People leaving or coming? Furniture rearranged? New furniture? Any strange cats outside? Something is upsetting him so you'll have to put on your Sherlock Holmes cap.
Years ago, after I put in a ceiling fan, my, seemingly laid back, cat hid for three days. When he finally came out, he would only walk along the walls, belly to the floor. It took several days for him to calm down. Sometimes, even when he was an old man, if there was a loud noise outside, or an earthquake, he would look up, suspiciously at the fan!
Anyway, back to your guy. For now, I might put him in one room, with his box and a bed where he can feel secure and protected. And clean under the bed really well with an enzymatic cleaner. Read the sticky at the top about "things that saved my house..."