It seems like he likes your mom's company , and I personally can find nothing wrong with this many folks have family beds, , however it may be a good idea if you spoke to your mom and as he is 11 years old and coming up to puberty ,get her to ask him to use his own room to sleep maybe he can pop in to see her at the weekends , I think for his own independance it may be good .,it should come from her, she can be firm but not create any feelings of upset ...