Can someone be trained to sleep on their back or stomach only?

I'm writing a book and in it the main character's stepfather has a crazy rule. "Sleeping on one's side is forbidden." The stepfather conducts several bed checks a night and roughly forces anyone who is sleeping on their side either face down or on their backs and shouts at him or her. The main character can't seem to get the hang of it,though. Is it physically possible to stay only on one's side or stomach?
 
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