Your question suggests that this happened to him at school. Handcuffing and interrogating a 9 year old!? Without his parents?! Is this part of the school-to-prison pipeline that is developing in this country?! Regardless, your question raises a lot of issues that should be discussed in the privacy and confidentiality of a lawyer's office. You should start with taking a look through any documents that you signed when you enrolled your child in the school. Read through all of the fine print. Is this a public school? Did they ever disclose to you, the parent, that your child would be treated the same as a street criminal, and that you would only find out AFTER the fact, from your child?
When I was growing up, schools had a much more effective and humane way of dealing with bad behavior: detentions, standing in the corner, forfeiting recess, and in worst case scenarios--paddling. Now they are treating children as if they are Bonnie and Clyde. Notwithstanding the gross misuse of public resources this represents, it is above all else, a very sinister reality that is developing in our modern world, and one for which society will pay a terrible price eventually.
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What you don't say is whether your son was arrested or charged with a crime (or the juvenile equivalent), or whether he faces possible juveniles charges later. It is possible that in these circumstances, your son's admissions would be kept out of evidence as involuntary and/or obtained in violation of Miranda. Obviously these are things to speak to a defense attorney with, preferably someone with juvenile court experience who knows specifically what is needed to present an effective defense in juvenile court. It's not wholly the same as in adult criminal court.
That should be your first concern, and even if there have not yet been any charges, an attorney could be helpful in sparing your son from charges and possibly even preventing a recurrence of this kind of incident.
You don't ask about a possible civil suit. Maybe there is a basis for one, but that shouldn't be your immediate concern. Protecting your son from the criminal/juvenile court system should be.