If you see him do it tell him firmly 'no' and take him outside. Don't make a fuss or he may do it all the more just to get the attention
If you don't see him doing it then there isn't really much you can do to stop him I'm afraid. Make sure that the bedding is washed properly in biodegrable washing powder, as well as the bed itself. If he continues to do it I would remove the bed and just give him a blanket that is easy to wash. Hopefully he will grow out of it.
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