SLD cannot be based on only one test, and while a discrepancy between index scores is often a red flag for learning disabilities, it is only a "flag". What were the results of the WIAT? If you have concerns about reading, did they conduct a reading evaluation? Usually this would include tests such as CTOPP, GORT, TOWRE, WJRM. They should also consider curriculum measures of reading to help assess how he does on grade level skills.

Here's a good link that discusses tests for dyslexia http://www.concordspedpac.org/Whichtest.htm

Also, keep in mind that a child can have a SLD but still not qualify for and IEP. In most states, the TEAM must find that the child BOTH has a disability AND the disability is causing them to make ineffective progress in the curriculum. Certainly if he has stayed back, this would be an indication of ineffective progress. In addition to this, one would expect to see low scores on WIAT and the other tests. Again, this does not mean that he would not benefit from specialized instruction - only that he does not meet the level of need to require an IEP to make effective progress.