Hi A.,
I am an advocate of keeping a child back WHEN NECESSARY.
That said, I personally wouldn't keep him back in Kindergarten - I would have him go through/attempt to go through first and if he is failing miserably, then have him either go back to K or repeat first grade. Your son is on the younger end of the spectrum - and boys mature slower.
I think the teacher your child has makes a HUGE difference in how your child does. If they don't mesh well, it's going to be a struggle. I speak from experience on this.
A short story: We moved out of state in April '06 when my son was in Kindergarten. 20/20 hindsight - I'll never move during a school year again! He did fine all of Kindergarten, and when we moved the new school wanted to keep him back.
I didn't feel that was appropriate at that time since they didn't have enough to evaluate him on, and I said move him to first and if he does horrible, I have no issue holding him back, but he needs a full year before a decision to hold back could be made. Fortunately, School Admin agreed.
He had to go to summer school (summer school for Kindergarten - sheesh! LOL), and they reluctantly moved him to 1st. He had a FANTASTIC 1st grade year, he is now in second and has matured so much since then. I'm glad I didn't hold him back. So in this case, we made the right decision.
Bottom line - this is your decision. DO NOT allow the school to bully you into holding him back. If you are opposed to it, you will need to argue your case. Look into summer school options for him. If he does end up staying back, it's not the end of the world, and it could potentially do him a world of good, as if he continues to struggle he could end up hating school.
Good luck to you!!