Thanks for the comments - we'll definitely look into getting him into a 6th grade math class - It is a strange structure - they try to keep 5th grade more like elementary school - a team of 2 teachers for all subjects for 2 classrooms worth of kids - which does allow for some ability grouping, but then they do the more traditional middle school switching by subject for 6th. We will schedule a talk with the principal/guidance counselor later this summer. When we toured the school, there was mention of qualified 6th grade students being allowed to take a compressed 7th grade math and Algebra 1 so they could start Geometry or Algebra 2 as 7th graders at the middle school. I will see if it is an option for 5th graders - or if he could take a test to place into it at the beginning of the year - I worry that it will make scheduling a nightmare though.
There is a gifted private school in town - I have a few acquaintances whose kids attend. One of the kids who also attended preschool with my son is there - and this other kid was also reading well in pre-K, had good fine motor control, better social skills (was also 10 months older), but from what I saw didn't have the crazy math thing. We actually had my son tested at age 4.5 and applied to the gifted school - submitting the test scores and a portfolio of his hand drawn calendars and self made math problems too - but he wasn't admitted. It still irritates me a bit - he was reading at a high first grade level at the time, and who knows where the math was - he was finding prime numbers. The problem (I think) was a bad site visit (we had to let him attend for a day) - he cried the whole time. He could barely hold a pencil and didn't really like talking to other children - so they told us he wasn't ready for kindergarten. (He has a summer birthday, and many people asked us / suggested we hold him back a year! I couldn't think of anything more tormenting than another year of preschool.) So . . .I'm hesitant to investigate the gifted school again - fairly or not.
I don't know whether it's worthwhile getting him another IQ test or not - I don't see how it would change anything. I'm most worried about the next 4 years - grades 5-8 and making sure he is appropriately challenged academically. We have a good high school, with lots of advanced course offerings, and the local community college offers dual enrollment courses, on the HS campus for many subjects or at the CC itself if he is done with the math (2nd yr calculus, I think) by 10th grade.