If you've installed custom firmware, you can update without any problems.
If you haven't and still plan on doing so, keeping the console offline is your best bet. Deleting the WiFi network is the easiest way to make sure it doesn't update. I don't think 3DS online services are still popular but your son can probably tell you that.
Hacking the thing took me two or three hours in total, with no hardware modification using the guide on https://3ds.hacks.guide/. Most of that was me being careful while reading the instructions and making backups, I'm pretty sure the entire process can be done in twenty minutes. Now that the eStore has been abandoned, it's the only way to play some eStore only titles so it's worth doing if you want to preserve that history.
If you haven't and still plan on doing so, keeping the console offline is your best bet. Deleting the WiFi network is the easiest way to make sure it doesn't update. I don't think 3DS online services are still popular but your son can probably tell you that.
Hacking the thing took me two or three hours in total, with no hardware modification using the guide on https://3ds.hacks.guide/. Most of that was me being careful while reading the instructions and making backups, I'm pretty sure the entire process can be done in twenty minutes. Now that the eStore has been abandoned, it's the only way to play some eStore only titles so it's worth doing if you want to preserve that history.