I think that there is something wrong with your x. What grown women in there right mind son or not would sleep in a bed with a 15 year old boy going though puberty. If I was you and I even think a tiny bet of child abuse I would call CPS. She seems like she is hiding something and your son is protecting her. I wish you the best of luck. Just follow your gut on these.
ummmmm the one thing that got me about this is your x tells you to mind your own business about your son?
This isn't normal, and I would be concerned about this. I don't know too many 15 year old boys that want to sleep in the same bed as their mother? I remember when my brothers were 15 years old, and they were close with my father and not so close with my mother.
I really don't know what to tell you about CPS though. If you feel your X is abusing your son, then as a parent you need to. Remember though that this could cause troubles between you and your son though, so you need to make sure with no doubt that something isn't right.