If school start times are staggered, getting him bused to the middle school should be easy - he just gets on the middle school bus instead of the elementary school bus. I don't know your school district, so I don't know if there would be issues getting him from the middle school to elementary after the class is finished. If you or someone else could drive him, that might be an option. If there is a lack of funding for the math, taking the morning bus to the MS, then sending a district van/car to transport him may be cheaper than the one-on-one teacher option.
As for the environment, if you had considered sending him to a high school, that would be more of a worry to me than sending him into a middle school GT class. By and large, middle school GT kids are good kids interested in academic pursuits.
As for private school, there are lots of them around here. About 30 to 40 percent of kids in our district and the neighboring districts attend private school. It is interesting to note that these three districts are always among the best in the state - that is, good private schools seem to exist in the very districts where kids don't really need that option to get a good education. I just checked the math offerings of the school I attended (private, all girls). They offer the same course sequence as the public school - goes through Linear Algebra and they both offer statistics (though public offers AP Stats, private is a one semester course). Most of the private schools around here are great, but this one has the highest average SAT scores, and they don't offer anything above and beyond what the public school offers. Not worth the expense for just the math offerings. You could even hire a private driver to transport him from MS to elementary, and that would be a lot cheaper.
