I hope it's okay if I weigh in here; granted this is a Dad's thread but for a different perspective and some food for thought. Seems like there's a couple of things to consider: first, you mention that he's apparently not along the path to puberty just yet. That's okay, as you noted. It was similar in our family. Every one was behind (physically and emotionally) and they came by it genetically as my spouse and I were 'late' bloomers. I suspect that when the penny drops, as it one day will, he will most likely become more private about his personal space and what he does within. Second, and this is somewhat anecdotal so YMMV, when I was undergoing treatment, I was quite ill for a number of days each week and this lasted for quite a while. It was a scary time for our family but something I noticed was that for one of my children the need to be near, to reassure(?) himself that I would be okay or that I wasn't lonely amid the hustle and bustle of family life (if I was resting) was apparent. He would 'nap' not too far away or come and give a hug or pat my hand. At night, no, because Dad was there but after school or on weekends when everyone was running around being teenagers. There was absolutely nothing sexual about it. I get a little choked up about it now because seeing him model the empathy and kindness of his father is something of which any mother can be proud. If he chooses to be a dad someday, he will be a wonderful one I know. (I should add that it's not that the rest of the family are cold-hearted grinches but he just 'sees' people very well). Finally, I don't know your dynamics specifically with mother and son. If he is sleeping in the same room of his own volition w/o encouragement then I suspect it will, as noted above, pass as he matures. If it would make you feel better to discuss it with her, good luck. I mean, that could go south real fast depending upon how it's broached you know? Regardless, talking honestly about your fears (and that seems to be a little of what this is, or am I reading it wrong?) about his potential discomfort or embarrassment seems like a starting point. Maybe she has a few of her own, ya know? Mixed feelings about watching your babies leave childhood and enter the world of young adulthood: pride and pain go hand in hand as you watch them go out and conquer the world.
*Note: several replies while I was typing this, and one of them broached something I wasn't comfortable mentioning--that it may be comfort for her. Yeah, that's one thought I had and was outside the realm of my experience with my own children. You've had a different situation and some talking things over with a counselor may help him/her/you establish some healthy expectations/coping mechanisms/acceptable ways to provide comfort and consolation.