Without his grandmother's statement that his taking of her money was against her will and without her permission or authorization, no prosecutor will file theft or fraud charges against your son.
You may want to consider whether there are circumstances such as age or mental disability that would enable you or another relative to be approved by a court to act in his grandmother's stead with respect to her finances to prevent further losses on account of his conduct.
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Generally speaking, there are two ways to recover stolen goods:
1) Call the police and seek to press criminal charges; or
2) File suit in court and seek to press civil charges (difficult if not impossible against a minor.)
Are you asking for advice on means by which you can control your son? If so, you'll need to be more clear whether he is living with you; whether he is a minor (and how old,) etc.
I am not your attorney and am therefore giving only general advice. DO NOT rely on this advice to make your decisions. Please see a local attorney, licensed in your state, for further advice.