Your son sounds a lot like my 8 year old son. He is very good at math but just doesn't really enjoy reading except for Pokemon guides. He reads a bit above grade level (perhaps a 6th grade level when he's a young 3rd grader), but math is an entirely different story. In 1st grade he did a 1st grade math program and was really bored and hating math. In 2nd grade he easily went through the 5th grade math book (we homeschool, btw). At the beginning of 3rd we did pre-algebra and moved on to algebra by mid October. He's finding algebra pretty easy, so we're moving through it at a pretty quick rate. He's enjoying math a lot more now that he's out of arithmetic. We're planning to do geometry over the summer.
Since our sons seem pretty similar, I found your son's scores very interesting. We're still saving our pennies for testing, but hope to do it soon.
