First, with any behavior change, you should take your cat to the vet to make sure there's no underlying health problem.
After the vet has determined that the change is behavioral and not related to a physical problem, you can start to look at your cat's environment from his perspective to try to track down the change.
Understand that cats are comforted by things that smell like them (they have a much stronger sense of smell than we do and it's much more important to their understanding of the world). The litter box is definitely a strong personal scent (even a "clean" litter box will retain some of the scent to a cat).
So look around for things that may be causing stress for your cat. Some possibilities include:
- Any changes in the home environment (addition/subtraction of family members, furniture rearrangement, illness in the home, unusual visitors, unusual frequency of visitors, etc)
- Any changes in your neighborhood, since he's an outdoor cat (mainly here I'd look for a new outdoor cat in the neighborhood, but nearby construction or new neighbors who spend more time outside are other possibilities)
- Any changes in your interactions with him (new food, new water fountain or bowls, different mealtimes, etc)
- Miscellaneous - anything that causes loud, unexpected noises can cause stress, as can a lack of hiding places or safe pathways through the house
Once you have some ideas (or even if you don't!) there are some ideas that you can use to reduce your cats' stress level. If you can pinpoint a few things that could be causing stress, try ideas from the following list that are related. For example, if there's a new dog in the house, then installing elevated pathways (bullet 2) will help him avoid the dog and feel more secure in the house.
- Try providing different sleeping places. A cat's ideal sleeping place is warm (sunbeam, covered house, thick bedding), with a good view (think elevated - a wall shelf, a cat tree, or a wide window ledge perch), and often partially secluded (again, a covered bag/house, or thick bedding)
- Make sure your cat has pathways through the house so that he can travel through the house without confronting anyone (human, cat, or any other pets you may have). Usually these paths are elevated to promote a sense of safety in the cat. A good resource is Jackson Galaxy's Catification pages, but the main idea is to make sure there are pathways and no deadends).
- Reduce the competition for resources among your cats. Start feeding your cats separately to reduce the feeling of competition among them for food. Also add more water bowls/fountains to different locations in your house. Generally there's a recommendation to add litterboxes until you have one more box than the number of cats, but I don't know how that would be changed by allowing the cats outdoors.
- Make sure you spend enough time in interactive play (with each cat). This utilizes their hunting skills and helps keep their minds sharp. If another cat is bored and attacking the elderly cat, this will help reduce the number of attacks/harassment.
To end this novel, I'll add that the first winter we had Cat Genies we found Kendall sleeping in one. They wash and blow dry the PVC litter-pellets, so we eventually figured out that he was cold and put a heating pad in a hidy-box. Now we know the weather is starting to turn cold because 3 of our 4 cats fight over who gets to be in the hidy-box (only 2 fit at a time). So, sleeping in the litter box is not always an insecurity thing. In our case, what changed was the litter box and the weather.