If it does go bad for the kid at trial I think he has a good chance of a presidential pardon, or at lease commute the sentence. If Trump loses this will come as he leaves office, if he wins, it'll wait till any appeals have played out, why spend the political capital if he doesn't have to?They ALWAYS say that at the beginning, just like civil litigants always start with "My demands are non-negotiable".
As an attorney, he will be professionally obligated to present the client with any offer made by the state. It might be hard for the kid to turn down "youth detention until 21 or we can go to the mat with an adult trial and risk life without parole".
An attorney once told me about a case he was taking to the matt and would never plead out. A year or so later when it was resolved he told me what a great plea bargain he negotiated for his client.