Jennifer,
I would wait till he starts asking questions. Rebecca is a big part of your family, and is still with you. I would tell him (from my beliefs, yours might be different), when he starts asking the obvious, that Rebecca is his older sister that went up with angles to God to watch over him. It's very hard with children, discussing death. And it certainly is harder discussing a stillbirth or miscarriage. We had to put one of our dogs to sleep in April. Of course Anne-Marie who is 3 had a ton of questions as to where he went, how he got there, WHY, where is God, did daddy take him to God....loads and loads of questions I was unsure on how to answer to where a 3 year old would understand. I know you are probably wondering why I am compairing the loss of my dog to the loss of a child, but that could be because I thought of him as one of my children. I'm in no way trying to be insulting, please understand that. I am just trying to tell you how it was for me trying to explain "death" to Anne-Marie. It wasn't easy at all and you have to be careful as to what to say. You don't want to scare him making him think that God is going to take him away from his mommy and daddy. You don't want to say anything like, She was sleeping and didn't wake up for he may fear to sleep. Stuff like that. It's so hard with children, their minds are like girble wheels spinning a mile a minute. There are some books on how to talk to children about death. Here are a few books you might want to check out.
I hope I helped somewhat at least.
How to Talk to your Kids About Really Important Things
What's Heaven
Butterflies: Talking with children about death...and life eternal
Waterbugs and Dragonflies: Explaining Death to Young Children
