B.G.
I would agree with most, about keeping the new one with you a while until he is able to develop a schedule of some sort. You may consider moving you 20 month old and new baby into the room with the 3 and 1/2 year old since they will be the closest in age, and giving your nine year old boy a room to himself. I understand you have a 12 year old, but she is only one of two girls, and girls seem to have an easier time with this sort of thing. Nine and three is a big gap and will seem even bigger when its 13 and 6, at least when it comes to boys. Through puberty, he may need a little more privacy. Six, four and two seem to be more appropriate ages to be sharing a room, looking into the future of course. Just a thought.
Good luck!