I have a lot to say on this matter because we just went through the exact same thing with our son last year...
My son is 5 and will turn 6 at the end of July. I had assumed that I would put in in Kindergarten this past year since he was very ready academically and his b-day was in July. However, his preschool teacher from last year recommended that we wait one more year and start him in Kindergarten this coming year when he just turns 6. Socially he just wasn't ready last year. His preschool teacher last year has a son (now in his 20s) who was the same way. He was very bright and academically ready, but not quite ready socially. At the time she didn't know what to do so she put in in Kindergarten. His K teacher told her that she recommended that he repeat K one more year because socially he was having a hard time. She didn't want him to be bored so she moved him ahead into 1st grade. He ended up repeating 1st grade because he just couldn't handle it. Even to this day, she says that her son says he wasn't smart enough and had to repeat a grade. She tells him that it wasn't that he wasn't smart enough. He just wasn't ready socially. Now she says that if she had it to do all over again, she would DEFINITELY wait. So that is why she recommended that we wait with our son.
I didn't know what to do because I didn't want my son to be bored in another year of preschool. He was already starting to read and do simple math at the end of last summer. I talked to a lot of people about it including my neighbor who is a kindergarten teacher and my mom who is an early childhood development specialist. What I heard from most people was that if I had ANY question in my mind, to wait and start him a year later. That is what we decided to do. He is in preschool now 3 days a week and will start K in August. I cannot tell you how glad I am that we waited. I think he will do so much better starting in K in August.
Every child is different, but from what I understand, most boys mature a little slower. Now-a-days Kindergarten is like 1st grade when we were little. They have increased the standards so much that if they are not ready, they can fall behind quickly. I think it is better to wait, than to have to have them repeat a grade later on.
I know you will hear lots of opinions, including mine, but you should really follow your heart and do what you think is best for your little guy! God bless. *Ü*