M.M.
I would move him now if you can. We have had to change daycares in the past (and not by our choice, we are doing it again next week) and it is NOT easy on the child. Our daughter had many sleepless nights and loss of appetite after her past move. I think it will be best for your son (and you) if he has time to adjust before his world is turned upside down with a new baby in the house. We are in similar situations. Our daughter is 2 and about to start a new center and I am due with our 2nd child in October, so this situation is all too real for me. Good luck!