I'll throw in my two cents based on my 14 yr old's experience. DO NOT ALLOW your nine year old son to throw a curve ball in a game! Success with a curve ball at that age is addictive and it will become his "money" pitch. It is much more important to learn control with the fastball - four seam/two seam and a change.
My son has been fortunate to play "major" tourney ball since he was ten years old and I can't tell you how many 10 & 11 year old curve ball pitchers have fell by the wayside. The fastball pitchers who were around as 10 & 11 year olds are now dominating in tourneys all over the South.
Our best pitcher had a 76 mph fastball as a 13 year old and a killer curve that he threw hard for a strike. He hurt his elbow, throwing the curve in the Governors game last June (04) and still can't throw without pain.
My son who had demonstrated a respectable curveball at 10 years old was not allowed to throw it in a game until last year as a 13 year old. He was forced to perfect his fastball pitches and change up. When playing up in a 14 year old tourney against a good Texas team, the coach called four curve balls. He struck out the first batter with the curve, then proceeded to completely fracture the growth plate in his shoulder with the fourth curve ball thrown.
The orthopedic said the x-rays indicated a small older fracture in the growth plate surrounded by calcium build up. He concluded that my son had probably slightly fractured the shoulder a month earlier diving for a baseball. However, the stress from the curveball caused the fracture to split the entire growth plate.
It turns out that my son's mechanics needed tweaking, but the moral of this long story is that a curveball thrown by a young kid with less than perfect mechanics can lead to disaster.
Don't allow it to happen to your son.