Your son seems to have been in the wrong place at the wrong time. MIP requires care custody or control of the alcohol to be sufficient for a conviction. That means that simply being present at the scene without more is not enough for a conviction. Your son does not appear to have possessed alcohol under the legal definition. Just because there was alcohol in anohter boy's car does not translate to possession by your son. They must show that he had care, control or custody of the alcohol, i.e. it was in his car, on his person, they saw him throw it away when they arrived, he smelled of alcohol, made furtive movements, acted intoxicated, had actual possession on his person. Alcohol in another boys car is not sufficient unless your son admitted to possessing the alcohol. A lawyer can help you evaluate the evidence against him. In short this is not constructinve possession by your son if the alcohol was in another's car.
You should hire a lawyer to represent your son. It sounds like your son is not guilty of being in possession of the alcohol, particuarly because it was not in his car. The prosecution must prove that your son exercised care, custody, and / or control of the beer. It sounds like the cop just went ticket crazy on everyone because of the one boy's behavior (of consuming and apparently possessing alcohol.) Your son pouring out the beer at the cop's insistence does not make him in possession for prosecution purposes.