Well, those are very strong MAP scores, but I wouldn't necessarily panic, especially if you are in a good school district and there is some kind of GT or ability tracking. I think many parents make the mistake of looking at average (50%)scores and interpret that to mean little Susie belong several grades above. Personally, I find it helpful to look at the 97th percentile (or perhaps 95th if you want to be more inclusive) for acceleration purposes, assuming that you want your child to be comfortably at the top of the class.
A reading RIT of 240 is between 98th (237) and 99th (242) percentiles nationally for Spring 4th grade and exactly 97th percentile for Spring 5th grade. (These are 2011 norms since I can't access the 2012 charts without providing a credit card.) As such I think that a typical GT/honors curriculum that works a year above would be perfect. In general, as long as there is some ability tracking, he will probably not be too out of sync for reading/language arts in middle school.
As for math, he appears more ahead there. A math RIT of 255 is not only 99th for Spring 4th grade, but is 98th for Spring 5th grade and 96th for Spring 6th grade. With those numbers, he definintely should have access to 6th grade curriculum and possbily 7th grade (pre-algebra) in the fall. Again, a lot of it depends on your local situation. A typical GT or honors middle schoool curriculum will be one year ahead so he could have access to 6th grade curriculum in 5th grade in the fall. You need to speak to the administration at the middle school to get accurate information on what is actually available.
Again, opinions will vary so obviously take it with a grain of salt. My DS will be studying algebra in 5th grade next year; as a 4th grader, he scored between 97th and 99th percentile for eight graders on several different tests.
