Hi Smargie, welcome to the board. First off, the reading scores are still quite good, 85 percentile for 5th grade spring. See Appendix B here:
2011 MAP norms That said, I would be concerned about the scores going down. Have you asked the teachers about that? I wouldn't place this all on your son. Also, is the window for MAP testing still open? I would ask if he could restest now, if you think there's an opening in the accelerated cluster program and if you believe your kiddo belongs there. The trouble with many of these programs is that they are not open for all who need it - many times they are limited to space available. So kids who need the program miss out.
When you say that this is the first time your son's reading MAPs are not off the charts, do you mean this entire year at the school, or this most recent test? If he had qualifying scores for the program earlier in the year, I would argue that the school should take that into account. One fluke test shouldn't be what is considered as representative.
On the other hand, maybe he'll do fine without the accelerated cluster. From what you've said, it sounds like this year has been challenging. Do you think it will be less challenging next year? And if it isn't, is the school willing to be flexible to provide more challenges?
