We can hope to raise his grade. But teaching him more vocabulary words or drilling him on the parts of speech will not open up new vistas for him. It is not within his power to learn to follow an exposition written beyond a limited level of complexity, any more than it is within my power to follow a proof in the American Journal of Mathematics. In both cases, the problem is not that we have not been taught enough, but that we are not smart enough.
I prepared a short response about how there's problems with IQ testing and intensive education can raise IQ, but then I stopped to think -- maybe he has a point. Maybe we have to face the fact that many children are not going to be able to handle high-level topics.
After thinking it through, though, my belief is that the root of the education problem is not intelligence. It has to do with overwhelming distraction, poor teachers, poor funding (yes, smaller class sizes do help significantly), and poor nutrition. The solutions are parental involvement (or something equivalent), merit-based teacher pay, smaller class sizes, and better nutrition. Saying that the children are to blame is not only wrong but counter-productive. I've never taken an IQ test, but I know that I'm no genius. My mother is a teacher (focusing on early childhood development) and my father used to debate issues with me at the dinner table.