Your son absolutely did the correct thing by pleading Not Guilty to the theft charge. He has raised defenses which could lead a jury to have a reasonable doubt as to whether he had anything to do with 'taking' the car or had any knowledge that it had been taken illegally.
Even if the evidence does not support him as strongly as you hope it will, he needs to allow his attorney the leeway to obtain as favorable a plea bargain as can be had, and the only way to do that is to plead Not Guilty and let the attorney investigate the case fully. This is particularly true in a case such as this where the first offender statute comes into play.
What he needs to do now is allow the attorney to do his job by investigating the case, and give the attorney all of the cooperation and assistance he can to do so.
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