For a brief time, I worked in Early Childhood Education and one of the things I learned was that many children - particularly boys - are simply not ready for our structured school environment until age 6.
This has NOTHING to do with their intelligence or future "success" (whatever that means). A child who is ready for K at age 5 is NOT more likely to succeed in life than a child who is not ready for K at age 5.
The university dean of the college I worked in was a big proponent of encouraging parents of boys to hold off a year before starting their children in Kindergarten, unless of course it was obvious the child was ready.
He started K at age 5 and struggled with school for years. His K and 1st grade teachers wanted to hold him back, but his parents (out of THEIR shame) refused to. Finally, in the ninth grade, he just couldn't keep up and the school forced him to go back and repeat 9th grade. At age 14, this was very traumatic and he still carries the emotional scars. It would not have been traumatic at all if his parents had held him back in K or 1st grade.
The good news is that today, he has a Ph.D., enjoys his work and is obviously a very intelligent man.
I am in no way implying that you are doing the same thing his parents did!! Quite the contrary - the fact that you are asking this question means you are putting your son first.
I only mention the above story because, if you do decide to delay K for your son or have him repeat K, it would NOT be an indication that your son is somehow less intelligent or capable than his peers, who truly are ready at age 5 for our educational system.
Best of luck with whatever you decide to do! M.