Cat started sleeping in a litter box

Hello, my cat is almost 15 years old. I had to go away for 2 months and he is now staying with my daughter and her dad, and 2 other cats. He is not a big fan of the other cats though. He is allergic and has to eat only his hypoallergenic diet, so when my daughter and her dad leave the house they leave him in the room with the door closed so that the other cats don't eat his food. However, recently they started to find him deeply asleep in his litter box, no matter if it was clean or not. When they are at home and on the weekends, he doesn't do it. I have now been away for 3 weeks and I am worried about this behavior change... What could this be and what can we do about it?

Hi.  Litter sites are an important resource for cats.  That is his territory - perhaps the most important part of it - and it has been picked up from his home and placed in a territory owned by two other cats.   I think the best thing for this cat would be to be back in his own territory, where he feels in control - as soon as possible. Meanwhile, be aware of the signs of cats becoming blocked, because this is a common sequale of social stress.  Also be on the lookout for other cystitis signs and eg pancreatitis or joint disease, which might make squatting to urinate physically painful.