What country are you in?
In the US most preschools don't 'test' kids, in part, I think, because they expect preschoolers to be 'all over the place' developmentally - which can work in favor of our kids.

I would say that it's entirely possible that Kindy would be a better fit for your child, but if you think so then go directly to your local Kindy and tell them your concerns.

It's also possible that preschool and kindy (especially if it's a half day program) will work well for your child, and skipping 1st is the way to go, if it's needed at the time.

Do monitor your child carefully, because spending long hours with kids who are at a very different place developmentally for your child, and teachers who are expecting your child to be quite different from who he is, can be very hard on certian personality-types of kid.

He probably will be even farther ahead academically by 5.5 years old, I'd say it is more of a 'promise than a threat.'

We keep hoping that the early developers are just 'early' and that if we wait, that soon the other kids will catch up, as they did with their other milestones. This must happen sometimes, but I wouldn't plan on it.

My guess is that if he's physically behind his 'intellect-mates' I would try and get physical therapy or occupational therapy, as it will be easier to 'catch him up' or hothouse him physically than it would be to 'slow him down' mentally.

If OT and PT aren't possible, and your son isn't also 'below average' for his agemates, I would try hothousing at home, with a book such as:
http://www.amazon.com/Ready-Activities-Grades-Physical-Education/dp/013673054X/ref=pd_bxgy_b_text_b

or
http://www.amazon.com/Out-Sync-Child-Has-Revised/dp/0399532714

Being competent physcially takes lots of practice and experience, and it can be sort of boring for some GT parents and kids to do the repetitions that it takes when there is so much else fun to do in the world. But if I had it to do over again with my DS15, both of us agree that we would be 'less child led, and more Adult high expectation' in the physical activity department. He's promised me to take the theoretical grandchildren out to play catch every single day after work, G-d willing.

Hope that helps,
Grinity

Coaching available, at SchoolSuccessSolutions.com