Thank you for your advice and quick response!
Toddlers do not adapt easily to change, so it's possible that the alteration of the physical environment has been unsettling. However, three weeks is not a long time. Stick with the usual routine, and don't 'rescue' him when he wakes up at night. He has to learn to fall back to sleep on his own, and he can only do that with experience. you're the parent, so you will do whatever you think is best. My guidance, though, is absolutely to not have the children in the same room. Starting school is enough of a change for now. As is often said, "If it's not broken, don't fix it."