They cannot offer a grade skip just in math(to attend math with 6th graders) due to scheduling issues...
FWIW, this was the excuse our school gave us for not subject accelerating DS in math last year and most of this year, something he badly needed. DS now misses some of his regular class to go to a different grade for math (after the school tested him and realized that yeah, he needed more challenging math). True, he misses some of his regular class, but he's not actually "missing" anything, since he's way ahead academically.
Something to think about when talking with the school. If you think math acceleration would be good, ask what your son would be missing by doing so.