Unfortunately, the courts in Georgia have said that a child as young as even fifteen can be questioned without a parent present by police. The US Supreme Court has been a little more strict about those requirements and it is never a bad idea to raise it as an issue in court, but I think the current status of the law in this state would say that what they did was OK.
Your son should get a lawyer as soon as possible. Like I said, just because the law in Georgia says one thing now does not mean it can't change and I would certainly encourage your son and/or his attorney to argue that that's exactly what should happen in his case.
You might want to take a look at J.D.B. v. North Carolina (2011) 131 S.Ct. 2394, a recent US Supreme Court case addressing the interrogation of minors and the application of Miranda, and see if you can find any support in that case.
If you cannot afford an attorney, you should try to speak to the public defender's office in your area to figure out how he may get representation.
Disclaimer: No attorney client privilege is established by receiving an answer to your question on Avvo. This answer is provided for informational purposes only. If you have further questions, please do not hesitate to visit my Avvo profile or website -- www.AfterTheTrial.com -- to set up an appointment to talk more about your issue. As required by Rule 7.2(e), Alabama Rules of Professional Conduct, no representation is made that the quality of the legal services to be performed is greater than the quality of legal services performed by other lawyers.