Having just done the same thing nathansgran I found that doing what he normally does with Mummy kept my GS calm and happy.
DS and DiL had been brilliant at preparing him too, and he knew that Mummy had to go to the hospital so the doctor could give her the baby, and that Daddy would go too to help the doctor.
Luckily for us, DGD came on a day that he went to playschool, so that took the morning up. When I picked him up we went home to have lunch, and then went to the hospital to meet his sister (she obligingly arrived at lunchtime). He was much re-assured to see Mummy and Daddy, and excited to meet his sister.
As DiL had to stay for a bit longer, we left Mummy to 'have a nap' at the hospital, and then went to McDonalds to have a celebratory 'tea'.
Sadly, he didn't manage to stay awake long enough to see everyone when they got home, but he knew they would be there in the morning, so was quite happy for Granny to put him to bed.
I hope all goes well for you and yours, and congratulations in advance.
If you keep it 'exciting' and re-iterate what a wonderful big brother he is going to be, I'm sure all will go smoothly.
Enjoy 
Have you stopped buying papers?
Retiring and living frugally in money from downsizing after years of stress
). So, when he came with grandma and grandad to meet his sister he got the gift. I still have a picture in my mind of his little chubby legs as he walked down the hospital corridor holding my Mum's hand, and carrying the train case in the other. he is 20 years old now and towers above me, but he still remember meeting his sister and receiving his present that she had brought for him. he was 26 months when his sister was born. He had spent that weekend with his grandparents as we had a suspicion that his sister was on her way