do you mean he sits in it because he doesn't move very much, or because he is impacted &its all stuck in his opening collecting debris?
If he just sits in it you can bathe his bum in tepid water &it should come away from his hair reasonably easily.
If he isn't moving much it could be due to pain or injury issues &its uncomfortable for him to move. If this is the case he'll need a vet visit to find the cause &probably some form of painkiller (metacam).
If he's of a more mature age it could be impaction where the soft stools clog up the opening &form a solid mass of poo that will get stuck &will need teasing out gently with warm water &TLC.
This is normally a rather smelly process I believe but well worth it when cleared.
I've never had boys but I think they need regular checking of their bits for debris-hay -bedding-sperm rods &poops etc.
There is a link on here somewhere. I'm on my mobile so it doesn't post links easily but I'll have a look.
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