I looked up some things on Lexile's yesterday. Quite a few state's achievement tests correspond with lexile's, so a lexile can be received from your state test.
I also got Alida's Song yesterday. It certainly is a strangely classified book; 5.3 AR, 1460 lexile, and I found it in the children's section of the library! Many of Paulsen's books are in the teen section. I usually read his books before letting GS9 read them, so I read this one. I'm going to ask the library to put it in the teen section. I'll allow GS9 to read it but I don't think the book is geared toward elementary interest level. I'm not sure what earned it the 1460 lexile. There are some foreign words, the main characters were Norwegian descendants.

Anyway, it appears Lexiles are a better guage of a child's reading ability than AR, so acs, your son is doing quite well. I'd think that's an additional confirmation that the school and you are doing a good job with his educational opportunities.