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 :)