If the kids are bothering him at school, tell the school resource officer. Because he is a potential witness, they can get in a lot of trouble if they decide to try to intimidate him because of the case.
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As a former DDA and asst. supervisor of the Juvenile Unit in Orange County, bullying is a serious thing and may rise to a criminal violation of Penal Code section 422. You and/or your son should definitely report this to the school and the SRO. Also, Marsy's Law is an amazing victims' rights protection you should look at.