Normal poop shouldn’t stick to a rabbit’s bottom. It sounds like he gets poop stuck to him not only at night, which could be cecals, but during the day when you are gone and he is caged. Is he a good hay eater, or does he prefer pellets? A diet too high in pellets and not enough hay can result in soft droppings and poopy butt. It can be dangerous to take a rabbit outside for various reasons, but you certainly don’t want to do so if one has poop stuck to him. That is a magnet for flies
Did you start using a different litter? Is his litter box an adequate size? You didn’t mention anything about peeing. Is he using the litter box for that? Does he have any kind of mobility problems that would make it difficult for him to jump into a litter box? Is his cage a small one? If it’s too small, maybe he feels like there is no separate bathroom area in it, so he just goes wherever he is sitting. A larger cage or an x-pen might help if that’s the problem. Were there any other changes in the past year that could have caused him to stop using the litter box?
Sorry for all the questions, but it’s hard to know how to possibly give you some advice without more details. You also didn’t mention his age or if he is neutered. It can be difficult to litter box train a rabbit that isn’t spayed or neutered, but it sounds like he did use the litter box before.
If he has an extra furry bottom, a vet can give him a hygiene shave so that poop wouldn’t stick to him so readily, but you do need to find the cause of sticky poop if it’s not just his cecals sticking to him.