Hi, Catalana -
Sorry for the slow reply -- real life got in the way.
About RHRW book 1 vs. book 2: I've only seen book 1 on Amazon, so my comparison is limited, but judging by the pages I could see, I'd say the difficulty level of 1 and 2 is pretty similar. You might want to take a look at both books on Amazon to get an idea for yourself.
About EXPLORE scores: I know the scores of one of my students -- my son (also an avid reader with a big vocabulary). The year he used RHRW book 2, he was 9 and got a composite score that put him in the 99th percentile compared to 8th graders -- sounds similar to your son's scores. So maybe RHRW (either book, since they look similar to me) would meet your son's needs.
