I have a step son who was 10 when we told him. I married his Dad, with his Mother absolutely hating our guts and everything that we stand for.
So I was beyond scared, terrified, he'd hate me, and then the Mother would tell him that's it, they'll never want to see you blaaa blaa.
Anyway, I was feeling poorly and it just didn't seem fair for him to think every weekend I was poorly, so we took him on a day out and his Dad told him.
I was crying because I was so scared he'd think I was taking him/his dad away, like you Alison Jay, why would he want a bro/sis. etc. When he's happy as things are?
He just sat and said nothing [ which was a total shock !] When asked if he was ok, he just gave us the biggest hug and said he was so pleased to have a bro/sis on the way. [Total Relief!]
I know it must be such a horrible thing, your so pleased, but in the back of your mind, it's like, OMG !!Is every one going to be ok?
But we have tried soooo hard to include him in everything.!! We have had a garden office built so the 3rd room is now the nursery, so he's taken to knocking up a few bits from the left overs and taken to build the baby a aeroplane , boat all over the summer and never chissled in his life and loves it!
So of course, that's going in the nursery [ Health and safety conscience as it's not beautifully finished, but I think that's his way of getting ready.
I dont know if that's something you could look into? Maybe getting him to be chief Big Brother, decorating or something?So he feels a real part of it?