There could be many reasons. If he is old, could he have an arthritis type thing? Or something else. Could be bully cats or turf war.
My cat was probably older than we thought. He started not going out much. He got some steroids and that helped him recover a bit. Move on a few months and he was starting not to leave the sofa. Eventually vet diagnose an cancer in the mouth. He had also had difficulty in eating before this. I hadn't realised how it was only the mushy food he was able to eat and he liked a bit of milk. There were other symptoms in my case. I felt really guilty after his diagnosis as I had put him outside for some air and he was always straight back to the window to come in. He had always refused to use the cat flap.
Yours could just be under the weather if it is for a short spell. It is beginning to get chilly. Isn't it catch 22? Don't they need to be outside to feel the cooler temp in order to stimulate the growth of their 'thermal underwear hair' for the winter? Hopefully, yours will just get over this bug or whatever.
He is 6. Yes he has a thing going with next door cat who hates his guts. He is eating a lot and sleeping a lot. He is an arsehole to us around 5 am, chirping, jumping on my stomach (he is 11 pounds) because he wants someone to escort him to the food bowl and top it up. I put him outside this morning but he came back after less than an hour...I'm worrying he is holding his poo. I need to take him to the vet.
Friend's cats currently only go out for a short while, and seem to be doing their poo in that time as no health issues (well not that kind anyway) and they rarely use their litter tray. They are tending to go out less as the weather is getting colder.
My (indoor) cats can sneak downstairs, use the tray & be back upstairs asleep again in under 5 mins - I only realise they've been down because the smell sneaks up behind me and grabs me!
I wouldn't worry unless he seems unwell. It's winter, sounds like he just prefers the warm. My cat significantly reduces how much time she is outside this time of year and I couldn't tell you how often or when she poos. As for the waking at 5 am for food, I would most definitely tell him to bugger off. Hes being lazy and greedy, just like most of us in winter 😋
Stay at home cats last much longer than the ones who wander over a big territory, with all the inevitable dangers. He won’t want to go out if next door’s cat is going to attack him. My cat only goes out if the dog goes too, to protect him.