M.B.
Hi J.,
I would start now. Talk to your son about moving into the bigger room and let him know that when the baby comes he will get to be the big brother that will help the little one learn the routines.
Let your son pick out paint and bedding (or atleast pick from one of two you have decided on). Have him help arrange the room and ask him where the crib would work best(if you dont want to put it up yet when it gets closer let him help). That way he can start getting use to the idea of the baby in his room.
Have fun with your room project! Best Wishes on your deliver and new baby!