Sorry, I laughed harder than I should have just imagining the whole thing.
Thank you for telling your story, I'll think about your rachet strap whenever I remember about my Coke can and that will definitely provide some comfort knowing there's just some crazy nonsensical people out there.
In terms of keeping him safe online look into some content filtering for the whole home network, either paid or self-hosted.
Projects that act upon physical stuff are the most rewarding. Something simple such as building a simple Python script that interacts with a smart home API and turns switches on/off or changes light colors around the house are relatively simple and require no hardware/mechanics if you're not into robotics, soldering or circuit boards.
You've got a great kid :)