Miranda is not required if no questioning is done. I would need more information to determine whether the search was legal. Most likely they smelled Marijuana which is enough for probable cause.
Your son needs to hire an experienced juvenile attorney. Especially if he has other pending charges.
I agree with Mr. Smith, Miranda only comes into play with questioning. With all he has going on, he needs representation to assist, I'd call a local juvenile criminal defence lawyer to obtain some assistance. Good luck, hope that's helpful.
Secure experienced counsel for your son. Sooner rather than later. Do not discuss the case except with counsel.
This is especially crucial for your teen, who is likely accustomed to sharing most everything with his closest friends. TroubIe is , well informed friends can become well informed witnesses.