CeeJay. Listen to Denny. Building a go kart frame out of electrical conduit is about like building a bicycle frame from plastic drinking straws. Goes together really quick, and looks good. When push comes to shove, it just isn't strong enough.
Also suggested by Denny, buy the 1st one. This is something I do for my grandkids. I have 4 karts. Three of them were real beaters. Doesn't matter if it even has a engine. Believe me there will still be plenty enough to do that for you and your son it will be a nice father and son project. New engine, seats, tires, and of course sanding and painting the frame. When its all done you will have something that looks and runs like new. BUT the most important part is that the frame will have been built on a jig, in a factory, out of the right material, square and true. And in the long run probably cheaper than building from scratch. Watch Craigslist, Offer Up, and Facebook Market place. All 3 of my beaters came from those places, and all 3 look and run great. FWIW, I couldn't weld if you put a gun to my head. And the 4th kart ( actually it was the first ) I bought brand new. 2008
still running strong. 19 year old grandson still has a ball in it.