At 9, I suspect he'll be better on an 18" bike (unless he is freakish tall!).
It's much better to get something in scale with the lad so that he doesn't have such a long reach to the bars, and the gait on the cranks isn't too long for his little legs.
So many parents come into the shop trying to get their kids on to 20" too early only for them to find that the kid doesn't use it or they come back for something smaller.
Pop in with the lad to have a go on both if you fancy - no obligation to buy anything, but I would really advise trying to find the right size first.