I agree with all the above posters - my son's school actually told us that he should try a different instrument at school if he is already advanced in a certain insturment. I tend to agree - because for an advanced child, such a situation would prove to be frustrating.
I have enrolled my son in a private orchestra operated by professionals who can instruct and guide him at his required pace and where he is amongst peers. Could you find a local Orchestra group that will help him participate in competitions? That would be an ideal situation.