Could you get a full grade skip? That would help as he could move with the 5th grade cohort.

My son is 10 and in a similar situation. He is 5th grade age and in 6th grade at a small private school that contains grades 6-12. He is further accelerated in math two years, meaning that he is taking Algebra I. One of the reasons we went with this school is that it has a high school. I'm not sure what we're going to do about his math on the other end, but we should have a 4-5 year run in the meantime.