Thank you for your replies.

I don't think that he was bullied by any of the others as he is still socialable with them all when they are in the room with him and no, he hasn't had any UTI's. Nothing seems to be different in the living room or kitchen. There is no indication that anything has fallen. When Buddy is in the room, Nugget rubs against him, so he isn't afraid of him and none of my friends or my sons friends have been in the house since about 2 weeks ago and I noticed that Nugget was acting this way on Thursday. The beginning of the week he was fine being in the kitchen and living room.
He came out from under the bed when my other son Mark and his girlfriend Kim, dropped by and he was rubbing up against their legs and when Kim got his string on a stick, he was playing with it like he always does.
He seems to just stay in my room, or just in front of my room in the hallway.
He seems even leery to go into my sons room which is just across from my room. He also seems to keep smelling the air. I don't know if that means anything or not.
I am going to observe him very closely for the next few days and see what he does. If he doesn't improve, then I will bring him to the vets, to make sure that it isn't anything medical.

Thank you all again and I will keep you updated on the situation.