It sounds as if the school doesn't accelerate, at least not in elementary. Here it is pretty common for kids to take the MS bus and take math 1st period, then take a bus back to elementary. Pretty common meaning that there are always a few kids each year. For the skipping 5th grade math and going straight to 6th grade math (but staying in the elementary school building), probably 20% of the 5th grade class does that.
If they just don't accelerate, I would think that there are other kids like your son. If things get better in MS, I would just wait it out. Not ideal, but once he gets to MS (in 6th, I assume), he may be able to accelerate or at least he can participate in activities such as MathCounts.
