As another alternative, is it possible for you just to keep him home and school him there? We faced similar dilemmas, and looking back on it, I think I would have been better off homeschooling for a couple years and trying B&M school later on.
I would look at your district Early Entrance policy. I think your child sounds like a good candidate, but it may come down to writing skills (neatness, etc). If they think his writing is not ahead, they may say no, even though he is barely younger than the entrance age for 1st. Girls may have an advantage here, as their fine motor skills are usually better at this age.
