How/what to tell my 21 month old about new baby

[Jojo13]

My son will be 2 in march and my second is due in 5 weeks.
I've had light-hearted 'chats' with my son about there being a baby in mummy's big tummy and he's patted my belly and said baby. He's still so little and doesn't really understand though, so not sure how much/how little to keep on about it..?!
Or is there any other way of explaining to him/preparing him..?
Thanks

Not talking from advice as my son is 5 so understands quite well but what about books about becoming a big brother, maybe if you read it at bedtime a few nights a week and he can see pictures etc, congratulations by the way :)