Good morning! I'm so sorry to hear about your grandson's situation!
Unfortunately, the state can still prosecute someone, even when officers failed to Mirandize him prior to questioning.
However, his attorney may have a basis to motion to suppress any information that came from a custodial interrogation
if the officers didn't Mirandize him.
A custodial interrogation basically happens when a defendant doesn't believe he can leave and when officers are trying to elicit information from the defendant.
An attorney's failure to act to suppress such evidence can, in some cases, potentially constitute ineffective assistance of counsel.
That can potentially provide a basis for appeal and a malpractice lawsuit.
Accordingly, your grandson may wish to have his appellate attorney review the matter to determine whether or not an appeal may be merited.
He should act immediately if he wishes to pursue that possibility, as the deadlines can be very short.