Eibbed,
When my older DD was 3, I checked the local elementary school and talked to some teachers and decided that Kinder would be a waste of time. My 2 DDs went to private preschool (pre K, K and 1st grade) and skipped K. When we registered, they have to take the test for grade skipping and start 1st grade at 5. It was seemless because we knew the policy of the school district. They were both happy in their 1st grade (for a while anyway). They still wanted to learn new stuffs and we did above grade level Maths at home.
It is real hard for the teachers to do differentiation in kinder. If your DS is at 2 grade above K, I would recommend applying for the grade skip (to the school). You don't have to dole out your money and some schools may not accept private testing result. They will let him take the test (usually IQ test and achievement test) and at least, you will know what his LOG is and can decide whether he should skip or not.
It's unfortunate that most Gifted programs do not start until 3rd grade. One admininstrator argue that the kids who learned early on were ahead in K but they will drop off and level with other kids around 3rd grade. Duh! If we are not differentiating, the gifted kids learn at slow pace (bored and frustrated), how are they going to progress? It's hopeless.
But DD8 is in 4th grade but her TAG Math is doing 5th grade level and DD10 (soon to be 11) is in 6th grade and her TAG Math is doing mid 7th grade and will finish the year at the end of 8th grade. Now, I don't have to do supplementing at home any more. If your school does real differntiation at 3rd or 4th grade, there's hope at the end of the tunnel.
Good luck!