Yes, these completes do have way cheaper frames an parts. But theres more to it. For example a cheap 300 dollar-ish fit TRL 1 has a hi ten steel frame what looks like a sheet metal sprocket a blocky ugly cheap stem and pretty much crappy everything. The 600 dollar-ish trl 3 has nice tires a nice stem, is chromoly etc.. both are made in taiwan. A frame made by Fit would be around 300 dollars and would also be chromoly. It would also be made in the US, making it in theory, better. (Especially considering its made in the US by S&M)
You might want to be realistic with your son and set a budget. Quite possibly, within that budget, a complete might be more bike. 500 dollars buys a pretty sweet complete. 500 dollars could also buy you a frame and some wheels, maybe a sprocket too? I'd rather just have a stock bike than an incomplete custom!
Seriously I wouldn't build a custom for a ten year old unless I really thought he was gonna stick with it or unless I had enough stuff lying around to build one relatively on the cheap. (I'm sure by the time I have kids I will be building them sick custom bikes)