From my perspective as a parent, there is absolutely no way I would accelerate for reading alone, primarily because you can't accelerate reading as a stand-alone in any meaningful way. A verbally gifted kid who likes to read can gain 4 or 5 grade levels a year, without any particular instruction - no matter where you place them at the beginning of the year, they aren't going to be reading challenging-to-decode material at the end of the year. As kids get older, language arts becomes more about writing (and the spelling and grammar needed to support writing), and less about decoding words. For instance, in 3rd, my DD's reading assignments were primarily multiple-choice questions demonstrating comprehension and retention. In 4th, she's being asked to journal her thoughts about each reading assignment.
Presumably there are opportunities in class to read material of the child's own selecting. Make sure there are books he likes to read, which is not the same as "books that match whatever reading level you've assessed him at." My DD tested at something like 10th grade equivalent at the midpoint of 3rd grade; it's rare I find a book much higher than 5th or low 6th grade that's interesting to her.
