my DS will also be 21 months when his little brother arrives ... he has no clue!!

we have got a couple of those books about getting a new baby, and he has a "baby" (but he doesn't play with it...) so we're focussing on other things he will need to know - like being gentle with other little people, etc ...

but nothing we've tried so far has helped him to understand he's getting a sibling ... yet!