These scores are pretty solid. It's hard to make the case that he has a reading disability when his reading scores are mostly in the upper quartile. His CTOPP scores, which are the underlying reading skills are not as high - mostly dead average seem to be a solid foundation for day to day reading. It would be interesting to see the CTOPP subtest scores in addition to the index scores that you posted. Sometimes there is a lot of scatter within an index that could indicate areas of concern. (such as nonword repetition - although still in the lower end of average).
It is heartening that his rapid naming scores are so solid. Often, kids with retrieval issues have trouble in this area, and this results in poor reading fluency. But, your child seems to have decent skills in this area.
So - as far as needing an IEP for reading - these scores do not support this. Not sure if there are issues with comprehension - this would be an additional area to consider. It sounds like writing is the real issue, and it will be important to tease out what are the contributing factors: handwriting, language retrieval and formulation, attention, organization all or any of the above.
I have no personal experience with IM - but have heard of others with similar profiles that found it helpful.
