Just a couple of thoughts: (but I'm not a teacher). It seems to be the reading that you focus on regarding your ds's ability. How is he in Maths and writing, compared to the rest of the reception children? Or if you don't know about the rest, have you bben told where he is n/c level-wise. It may be that he is actually doing really well!
How do you know that in reading he's not as far ahead as he should be? What stage is he on ORT/Ginn etc. It sounds nosey but once you've been through it a couple of times, you get a feel for where they should be, what's good/bad etc.
My dd moved to an independent school after reception and was definitely behind her peers when she arrived. However, I bought lots of ORT books from Amazon and she went up 8 reading levels in Y1. I later discovered for my son that ORT do fiction books which are not the Biff & Chip series, he loves these as he doesn't see them as reading scheme books. they are called Treetops.
My gut feeling is that a change of school probably wouldn't help, but there's a lot you can do at home to push him on if he is indeed underachieving.