Originally Posted by Chester's Slave
He's the only cat in the house. It's the same size litter box he's used before. And the box is where it has always been. he just started this about 4 or 5 months ago. As far as I can remember , there have been no changes in size, brand, location, etc.
Two other suggestions come to mind, then. One - is it possible that he is feeling stiff or getting a touch of arthritis and finds it difficult to turn around in a small area like a litter box? Does he appear less agile or flexible? Does he hesitate jumping up and down from things, or give any other type of appearance that his age (not that he is old, but he is approaching the age when things like arthritis might start) is beginning to catch up with him?
Is he overweight? Again, it might be difficult for him if he has a little bit too much weight on him to do the manouevering necessary to cover his poop.
You might try adding a second litter box for him and see how he approaches the new box. It may just be that as an only cat he knows you are going to clean out the box and so he has no need to cover it up.
Has his diet changed? Is the 'stinkiness' something new or is this something that has been constant? It may be that as he gets older his dietary needs are also changing and he needs a food aimed at senior cats that is easier on his digestion.