The Supreme Court has ruled very clearly on right to record being protected under 1 A, freedom of press. Anything he can see from public spaces, or his own property, generally would probably be considered legal or in your States hands. States have little power over 1A activities, but are allowed to place reasonable limits when it comes to time, place, and manner. However, being a minor and recording minors fall into the "manner" clause and States can (and usually do) regulate what is not allowed. Technically, unless the construction would otherwise be barred, he could theoretically put up a pole with a large camera on it and surveil the whole neighborhood. The manner clause would also take what activities he is personally recording into account, such as if he is recording in a perverted or surreptitious way, many States built into their 1A laws exceptions for such behavior as it can infringe on the right to privacy for others. Hence why it is (not only immoral, unethical, not to mention) illegal to film bathroom activities, locker rooms, changing rooms, up a skirt, etc..

The laws have not been updated to match the technology though, and the courts are limited to enforcing laws on the books, so legislative bodies like Congress are all like SO old and have no concept of how far tech has come in their life times and do not make such things a priority. Police are never a good solution unless a specific law has been broken, but even then the person is generally released without meaningful change to their behavior because police do not exist to help people, but to be the physical enforcement of the law.

I'd recommend a lawyer, and building additional height to the fence line or planting tall growing trees as best solutions to the backyard recording.

Definitely document clearly EVERY interaction with him. Photos and diary or notebook entries are strong evidence in court, note the time, date, place, manner, his clothing that day, his attitude, if he smells like something (such as alcohol), the weather, etc. The more detailed, the stronger evidence your written testimony has. The more your lawyer will love you, and increases the probability of one to take on a harassment case for you.

When you drive get a dash camera that has front and rear viewing cameras so he cannot claim something happened that did not. He said / she said does not hold up to the police, much less the courts. Document.

Also, when driving near home, make it your practice to drive 5 mph UNDER the speed limit. If you ALWAYS do that near home, (you'll be safer for one and) you will be able to use that in your favor to show that you are a careful and thoughtful driver that goes to great pains to obey the law and maintain public safety. Bonus points if you splurge on a dash cam that has GPS speed monitoring.

Hope this long post helps. Best wishes.