That would be me... after a month or two in kindergarten, the powers that be decided that I should go to 1st grade, as I could already read (I was reading the Little House on the Prairie books at that age) and write (this was a little more basic). It was stupid, because it's not as if I was challenged in reading in 1st grade either. End result? I graduated a few months after my 17th birthday. It never should have happened. Why? I was always behind socially and developmentally, and felt left out a lot. It got a little better in high school, but when your friends get their driver's licence in 10th grade, and you're still 14, or you date a guy one year ahead of you, and your mom won't allow it because you're 15, and he's 17 or 18, it causes a lot of teen angst. And there's enough of that without the added problems of being too young for everything in 'your world'. I'd imagine it would be worse for a boy, being the smallest in the class? Not to mention boys usually being less mature than girls anyway, he may have a hard time finding a homecoming date. And I won't even get into the problems of graduating, moving out on your own and being 17 until into the following calendar year. You can't sign a lease or any other legal document, it's just a mess.
Anyway, you have to do what you feel is right, but unless you intend on putting him in some kind of private school until college where this is the 'norm', I wouldn't recommend it. I wouldn't subject a child of mine to what I went through. Not that it was
horrible or anything, but it caused unnecessary problems, and who wants that for their child? Not to mention, the children who are 'advanced' in the (fairly simple) lower elementary grades, may not be later as other kids catch up. And what's wrong with letting them be the star of the class for now? It's fun, and a great ego boost for a little kid!