Well at that age, he is very impressionable, and I'm sure the Marine recruiters did a thorough number on him, so now he is raring to join up.   Therefore, he is unlikely to be talked out of it.  It would help him enormously to talk to someone who has served ideally in Iraq, or a recent conflict for an insider's view of the situation. 

If he did sign up now, he is likely to serve in Iraq, but I'm not sure that is guaranteed.  If he is young and idealistic, he will also be strong-headed, and perhaps not listen to anybody else.  He may even voice that he is willing to die for his country.  Very challenging situation. 

Gene