Nowadays kindergarten is what 1st grade used to be. I would let him wait one more year. My youngest, Dija, is a September baby and at the time I was young and naive, so I didn't even consider the possibility of letting her start Kindergarten a year later. She struggled through and has been challenged ever since.
Now my girls are in Waldorf (where Kindergarten is more like preschool and kids go into 1st grade when they are 7 not 6 because they pay attention to children's level of development unlike current public schools in my area...) Dija is doing so much better and is happier as a result of not being pushed so hard. She still has a stigma about her hand writing from being forced to do it earlier than she was ready for, not being taught how to hold the pencil properly, then given years of negative reinforcement about it being sloppy, but I'm hoping that will resolve itself in time.
Sorry for the soap box, I guess my point is, after learning about childhood development (I took a very enlightening class in college a couple of semesters ago,) seeing what our public school system is doing, and talking to my girls' teachers over the years, a pattern of pushing kids too far too soon is emerging which results in high frustration and low self esteem over time for students and teachers alike. If it were me, I would let him have one more year of preschool.