Urgent advice needed - new sibling about to appear!!

If he doesn't know by now what is going on you have a lot of ground to cover. My GD (nearly 2years old) knew exactly what was going on and had been regularly communicating with "the baby in mummy's tummy". When I stayed to mind her she knew where mummy was and that a baby would be coming home with her. DIL was only in hospital overnight, baby was brought home the next morning and I went home. You could look out some photos of him as a baby and tell him mummy is going to bring him a baby to be his little sister. MY DIL had a present from baby for my GD and one from GD for baby. GD played with both of course!

I told her the truth!

GS was nearly 3 when his brother was born. H and I moved into D's house as they lived an hour and a half away at the time. He knew mummy had his little brother in her tummy and had been told she was going to the hospital so the doctors could help mummy get him out (caesarean)
He was completely accepting of this as he had spent several nights at ours due to it being a difficult pregnancy. So sleeping in his own bed with nannie and grandad in charge was fine.

Mine was just two(exactly two years between them) she wouldn't have understood anyway.

Wonder what your DS and DiL has told him? Actually this happened to us too. Our GS then aged just 2 came to stay with us for , in the end, 4 days while baby produced then in hosp for 3 days then settling back home. I honestly can't remember what we told GS. The excitement of it all took over for him (and us). We were fully occupied keeping him busy and he was excited to be staying with us. He does think that hospitals are where you go to get babies though despite knowing that his baby brother was in his Mummy's tummy. Good luck. A busy and slightly worrying day ahead for you all!