I think holding your son back is a huge mistake. HUGE. Children need to go to school when they are age appropriate. Your son's birthday is JANUARY. The cutoff is likely September 1 in your area. By holding him, you would be putting him school with children who are up to 19 months younger than him. That isn't fair for your son or the other children in school. He needs to be with children his own age.
He will not benefit from waiting an extra year. He is going to be one of the older children in his grade even if you send him when he's supposed to go. By waiting a year, he will be much too old to be in kindergarten.
Also, having siblings in the same grade that are not twins is just asking for trouble. Why inflict the possibility for so much competition when you don't need to? Let them have their own identity, their own friends, their own lives. Don't hold your son because you want him in the same grade as his little sister.
Your son needs to start kindergarten in the fall of 2014. He will already be five and a half years old.
Your daughter should start kindergarten in the fall of 2015. Yes, she will be almost six, but per school cutoff dates, that's when she will be eligible to begin.
I find it incredibly frustrating and concerning that so many parents hold their children back. It does not benefit them and, down the line, often creates more problems when you have 19 year olds in high school. Children are meant to go to kindergarten when they are five years old. Not when they are six and a half.
Holding your son back is a huge mistake.