K.B.
My brother actually repeated Kindergarten. He really did not know the difference and since he "fit in" better with learning and maturity wise he excelled much more then he would have in his "original" class group. Looking back my brother was thankful for being held back and he was always a good kid, no rebelling.
To everyone I talked to I have never seen a bad long term effect on hold a child back when in preschool - 2nd even 3rd grade. They are still young enough that it really does not matter/effect them negatively. Holding them back after 4th grade I think will create more of an issue since being held back could seen by other kid as a negative thing and they already had a few years making friends in that class group. So holding back sooner rather then later is better.
My daughter's b-day is Aug 16th and when kindergarten comes I will be faced with the same decision but right now I think I will put her in when she just turns 5. She has actually already spent a week in preschool (even though she is only 2.5 years old) while I was painting a backdrop for the school's play. She was already leading and teaching the 3-4 year olds so she is seems very ready for school, but once kindergarten gets closer I will make that decision. Things might change.
Every child will be different so evaluate your child as best as you can, talk to his teacher(s) ask what they think and do not make a big deal about it when you tell your son (if you decide to hold him back). More then likely he will just think he is in another preschool class or different class. If you do not hold him back but at the end of kindergarten you think he might need to be held back that is still a good time to repeat kindergarten.
Best wishes in making your decision, whatever decision you make stand by it because then your child will see that this was done in his best interests (whether he is held back or going forward as is).