10 yr-old pitching Fast Balls

Question
I need to know if my 10 year old can hurt his arm, if all his coach wants him to throw are fastballs.  He pitches 3 -4 innings, and the coach only instructs him to throw curveballs or fast balls(the majority of pitches being fastballs).  I am concerned becuase many people have told me that this will hurt his arm, his coach disagrees.  Who is right?

Answer
Hey Grisly,

He would have more damage if he threw all curve balls. Fast ball and change-ups at this age are the best... 3 innings should be okay as long as he keeps it to about 40 - 50 fast balls per outing. The rest should be change ups and maybe 5 curve balls, thrown the right way.
Good Luck,
Dennis