Sounds like you need to see a behavioral sleep medicine doctor.
Borkus' idea sounds promising.
Not to draw any anger from you, but I had to chuckle reading your post as I was looking back at my own childhood... I went through some of the same things. Though I was a little bit younger than your son when I had these feelings, I had them none the less. I was scared to death to sleep in my own room at night for many years. I don't know what it was. Over-active imagination, or something. But, left alone, my mind would go into overdrive. But, to bring it back to topic. One thing that sprang to mind, would be to buy a cheap set of walky-talkies. The ones like they have at Walmart. Preferably, something of the nature in what he can associate with. Transformers, or whatever it is they follow today. But, give it to him and make a game of it. Try playing with it, at first. Play with him in the daytime, making him go somewhere where he can't see you, but give him directions to come to you. Do this for a few days, or however long it takes for him. And, have a talk with his older sister. I'm sure that she would like the personal space, and perhaps give her a set as well. And, when you think there is a point where you can tell him, "Ok, (insert name), you're going to be a big boy tonight and sleep on your own. We have these, so you can talk to us anytime you need to. We are right here". And possibly, get him something that he can sleep with. A sizable stuffed animal. It's likely that he just needs to be able to feel something next to him to feel secure. Not sure if this will work, but I hope it does. In the vain of trying to help others, I offer to you what helped me when I was six years old. This is what my mother did for me.