We have something similar with our 6 year old. He is scared of being left alone, terrified that we will leave the house and leave him alone. One night, months ago, I was out and DH had put him to bed, then popped down the garden to get something out of the shed. He was out of the house for 2 minutes, max. In that 2 minutes, DS1 got up and came downstairs, found the house empty and screamed the house down. DS2 woke up, discovered that the house was empty and big brother was beyond terrified and also began to scream. I walked in through the front door at this point to two terrified, screaming boys, just as DH walked in through the back door.
DS2 was so frightened, he vomited all down my back (nice). DS1 took the best part of an hour to get back to bed, he kept remembering the fear of an empty house.
And since then, bedtime for DS1 has been a disaster. We've tried homeopathic stuff (drops in his bedtime milk, a roller ball thing on various pressure points - I know all that stuff is a bit woo, but I thought if it was a relaxing smell or something it might help), which I think helped for a bit, in a placebo sort of way, but was too expensive really to keep it up. We've got an audio book he listens to, I stay in the room for half an hour after he's gone to bed (but I refuse to stay any longer than that), he's got a nightlight... we've also incorporated locking the front and back doors into the bedtime routine and I will often put my pjs on at the same time as him so he knows I'm not going anywhere.
Hardly anything works. The only thing that makes him stay in his bed is if I either stay in the room (but then he wants to chat - that's not happening) or if I go to bed myself. I only do that on nights when DH is out (I love to go to bed early with my book!), so if DH is home, we get a lot of DS moving about, coming downstairs, getting in his brother's bed...
I have taken to putting an audio book or a DVD on in my room so it sounds like I've gone to bed, but then sneaking downstairs. However, I fear this will backfire in a big way if he gets up and I'm not there - I suspect we'll have another screaming session with him thinking I've left him again.
I am planning to move some furniture around this weekend and try something new with where they sleep - they share a room - and possibly putting DS2 on the top bunk (but he's only 3 and wakes in the night, and I'm scared he'll fall even though he is perfectly capable in the day time...).
It's really hard, and I get cross too sometimes, because I know I'm not going out! But that night really messed him up even though DH was only in the garden (and it wasn't even dark!), and I know that we have to get him to a place where he is ok with being alone (even though DS2 is in the room, it's really not the same thing).
If I was a single mum, I'd just stick him in my bed, but DH won't allow that, so we have to keep trying!
Good luck OP, try not to feel too bad about it, loads of kids play up like this. I used to babysit a little girl, and I'd tell her that I was just popping downstairs to tidy up and that I'd be back up in half an hour, and I never went back up. She never knew, she was asleep. (This never worked with DS1 - he just waited and waited and waited and then came to find me).