I agree with Lola, W needs to realize that when she moves out things change. It will be your time to have a bed-time routine with your son.
Will you be granted same privilege to go over to W's new place to help put son to bed?
Speaking from experience of being separated it will be hard, but take each day as it comes, and it will get better.
I have a 5 year old, and he does not really understand, but still knows that Mommy is not there some nights.
Me: 41 W: 41 Married: 17 years Together: 19 years 16-Sept-2008: "W: I want to move out." 16-Jan-2009: Separated, wife moved out. 31-Mar-2010: W, and kids move back home! D 14 S 12 D 11 S 7