Special circumstances, what should I do?

Jessica
My first husband and I divorced when my son was 3. We are both remarried, and his dad and step mom (my sons) became pregnant last year. In Jan at 32 weeks, her placenta ruptured, and the baby didn't make it.
My husband and I are now expecting. We hate keeping it from him, but we are so scared to tell him, because he took the baby's death so hard. When should we tell him? I'm thinking after my first ultrasound when we get to hear the heart beat.