There are lists of nonsense words you can access online and you can try having him read them to see if he has any obvious difficulties. I gave them to kids in kindergarten or first grade when I worked as a reading tutor, and there were benchmarks that the kids were supposed to meet when reading these lists timed. The ones who couldn't do it got reading interventions. After 1st grade, the schools didn't do this anymore, they had the kids read "passages" and timed their reading fluency. I taught a 3rd grader who was clearly very intelligent but just skated by right below grade level. Not enough to trigger anyone to do an educational eval but when I listened to her read aloud I could tell she had some definite processing issues. She kept skipping words and could read some complicated words but got hung up on very simple words. I timed her reading a passage every week in terms of words read in a time period (1 minute, I think) and she was up and down on her graph wildly, sometimes she sounded fairly fluent and other times her reading was very choppy and slow. So she seemed to vary even from day to day depending on her emotional status that day or other factors. One day I even got out the K/1st grade nonsense word fluency list and had her read it and she could barely do it. Meanwhile no one in the school seemed all that concerned about her because all they saw were the numbers which indicated she was close to grade level. They never/rarely actually listened to her read. So I was unable to get anyone to evaluate her.
If the school will not take your concerns seriously then I would get your independent psych to do some achievement testing for reading and see what you can find out.
Here's one page that I found after a quick search of nonsense words but you may be able to find an actual Aimsweb assessment.
http://www.maryvaleufsd.org/webpages/kfrauenhofer/teacherresources.cfm?subpage=10227
