Re: young kindergarten - holding back?
Just wanted to add that it sounds as if he may very possibly be gifted. Not way off the charts gifted, but at least on the lower end. Not necessarily an issue now, but might be something to keep in mind for later.
I do think that at this point, putting him school would be best. Now, whether that is public, private, or montessori, I don't know because I don't know the schools in your area. But, if he is already academically at or above where he needs to be for kindergarten, holding him back a year is going to put him far ahead. Now, kindergarten is not mandatory anywhere in the US, as far as I know. If I remember correctly, children do not legally have to be in school before the age of 6 or later (depending on the state). So, you have the option of not putting him school at all, hoping he catches up socially, and putting him right into 1st grade in a year. But, you still have to figure out where to put him this year.
I am assuming that homeschooling is not an option? If it is most definitely not an option, then, I stick to my previous opinion -- put him in somewhere.