The 3rd grade teacher sounds flexible and strong which is certainly a great indicator.

It seems as if your choices for next year will be to keep him with the current peer group for one more remaining year (before they disperse to different schools ?) or introducing him to a new (similar ability) peer group which he may continue to be grouped with past 4th grade. I guess it will be a tougher decision if he meshes really well with his third grade class this year. Neato has offered a good possible third choice too!

I have found that having a strong peer group has helped keep my daughter academically motivated, so I have sought out the most advanced peer group and most rigorous learning environment available to each of my kids for this next school year. This plan requires a long and complicated commute each day, so it better be worth it!

If math and science are particular strengths for your son, you may want to look into the residential rules pertaining to acceptance to The Charter School of Wilmington sooner rather than later. The book Genius Denied has at least one chapter concerning that school and it sounds like entrance there would be an absolute windfall (for my DS, anyways)!