How do you tell your children?

I have a two year old son. When we first go pregnant we were excited and we taught our son that there was a baby in mommy's belly. He would point at my belly and kiss it. He had an ultrasound picture that was "his" that he hugged and kissed. I was 15 weeks when we lost our baby Riley. It's been a week and our son keeps pointing at my belly. And when he see the ultrasound pictures he points at my belly. I've told him that the baby had to go "bye-bye", that the baby went to heaven with God. He still points at my belly every day, I do my best not to cry infront of him when he asks. I just don't know how to make him understand that the baby is gone. I hate when he touches my belly now, it makes me so sad that I couldn't give him a brother or sister.