Finding the right words can be difficult. Often the simplest way is the best, but you can also have a bit of fun...
You could always go for "Close your eyes and count to ten" or "Would you prefer twins or just the one?" or "You're going to have a new job in a few months."
How about getting him a World's Best Dad T-shirt, or doing it with words? Even if he's not into books, find some quotes or poetry about fatherhood or babies.
Alternatively, take him shopping and hit the maternity section!
Telling your mum
Get her some wool, knitting needles and patterns, or a book about grandparenting. Why not give her a photo of you as a baby with a caption such as "Now I'm the one who's going to have to get up at all hours of the night..."
Telling your dad
Try "You've got nine months to practice your Lego skills" or "What do you prefer, Grandad or Grandpa?"