To be perfectly honest, I think you are doing all the right things. I would have suggested you do the low-key taking him back to bed, tuck in, quick cuddle and night night. I would also have suggested the night light.
It does sound like he's reacting to his little sister being born, perhaps he just needs the reassurance that Mum is still there for him, maybe he feels more vulnerable at night?
I can only suggest that you are consistent with what you are doing, gently and firmly, and he will be reassured that nothing has changed, Mum and Dad are still there for him, and it should pass.
The only thing I would suggest is to stay away from threats of doors being shut, etc. This will probably only serve to unsettle him, and may well prolong the issue.