Toddler scared of bedroom

My son is nearly 2.5 years old and has all of a sudden become scared of his bedroom (he tells us he's scared). He's always been an amazing sleeper and we had no problems when we moved him out of his cot and into a single bed. We can't work out what has caused this and how to help him through it.

I've tried a night light, leaving door open, white noise, comforters, new blanket, massage, calming oils in a diffuser but nothing helps. I have to sit in his room (I'm now sitting just outside his door where he can still see me) while he falls asleep but as soon as he wakes and sees I'm not there he starts screaming and runs to our room and wants to sleep with us. I take him back to his room and resettle multiple times a night but I'm 5 months pregnant and completely exhausted as this happens for his day sleep also. Has anyone been through something similar and can offer any advise?