I'm not going to worry about what section we're in. Unless you are a pretty decent fabricator, and have the ability to bend steel tubing and probably weld, building one is not going to be that easy. Something you may not want to hear, it might be better to look for a frame or complete kart that already has one. Lots of karts, older simple ones were manufactured with a cage. Not hard to find at all. I have
4 karts for my Grandkids and all of them have a cage. For the very same reason that you want one. Safety. Regardless, get a helmet for him, and make him wear long pants.