Don't feel bad! It might not be anything you've done at all. Maybe he's just having a rough time adjusting to the move/new house in general. I think that's only natural.
Maybe try putting him to bed to start the night in the big bed, so he's not in bed in "his" room at all. To reinforce that the twin bed isn't for him to sleep in if he doesn't want to, you could get one of those bed tent things, and make it a playspace.
We had a similar thing, where one night we tried putting ds to bed on the bed in his room, to see how he'd sleep there. The first night he went to sleep in that room with no trouble, with me laying beside him. But then he woke up screaming in the night, and of course came into the big bed. The next day I was wondering if he'd be terrified of the little bed, but he wasn't. Wouldn't go to sleep in there again though! I was relieved actually because I wasn't at all ready for him to move out. I was just worried I'd been waking him by my tossing and turning. But a week or so later he started sleeping through and hasn't looked back
No particular point to that story! Except that this will pass. Your son will forget about his rough time soon and will happily sleep in your big bed again. I think he's probably just adjusting to a lot of changes and it might take a little while.