He could try to guard you if he feels you are a valuable resource (you provide affection and attention, so you could be a pretty valuable resource to him). If your room is where you spend most of your time, maybe he sees that space as very valuable. I wouldn't say it is so much about any kind of dominance. More about he feels he has a resource (possibly you when you are in your room) that he feels he needs to control and protect from anyone who he feels might want to take it (possibly your son).
This is why I like the place or the down stay. This way, it becomes clear that if someone comes in, he cannot go up and try to control that space. Another thing you could do is if he growls at someone coming into the room, you could remove him to a different room and have a "time-out". This way if he is guarding you, he learns that if he tries to guard, then his resource goes away. It could be helpful, but it doesn't give you as much control over the situation and is more reactive than proactive.
It probably doesn't happen in other places of high traffic because they do not have the same association or the same value. My dog tried to guard the front door, but could have cared less about the back. They were both doors, almost exactly the same, but the front door was where new people came in. So that was the one he tried to control because he liked the attention he got from visitors and he did not want anyone else to control it.