One of my suggestions is to get a camera and one, perhaps, that records audio as well.
Also, see a lawyer. Restraining orders can be filed and won against people who are harassing you. Make a record with the police and if you are comfortable, perhaps a "friendly" letter to gramma -- assuming she is legally in charge of him. I have gotten clients civil harassment restraining orders and have had violators spend some time in county jail because they thought the piece of paper they were served with meant nothing.
Just a suggestion. Judges do like it if you try and settle differences with your neighbors and don't just run to court. Also, an attorney in your area may be able to let you know if there are any noise abatement ordinances. You can get someone, an expert, to come out and set up recordings that are admissible in court with foundational testimony, assuming there is an ordinances, that evidence the noise which is being made is in violation of the ordinance.
Best of luck. Don't let this kid, his gramma, and his pals bully you.
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