Something to do with how many word are on a page, difficulty of the words and how many they can read in a minute and so on?

As a side note: I have no clue where DSs reading level is and I am curious to pull books off of his shelf that I know he reads and compare them on the Lexile site!

No idea but I wanna know too.

I will ask a teacher at baseball tonight. we were talking about it not too long ago.

I found this interesting. It is a reading assesment but not exactly what your talking about.
http://highland.hitcho.com.au/readingassessment.pdf