I have read all the responses you've received and I have to say I disagree with those who are telling you to send your son on to K. I think you answered your own question in the first sentence of your request. If your son is on the immature side that is the very reason you should WAIT to send him to K. He wouldn't struggle academically but maturity level is just as important, especially as he grows older. Academics is not the only factor to consider.
We faced this same decision with our now 10 year old son who has a June birthday. We had him tested on two separate occasions while he was in 4 year old preschool. He excelled academically (off the charts) but lagged behind in maturity issues such as tying his shoes, dressing completely on his own, etc. Both testers recommended we wait to send him to K.
We talked with numerous parents who had faced this issue and teachers as well. Every teacher recommended we wait. Not one parent who waited to send their child to K ever regretted it. But many of those who sent their child ahead regretted it later. Their children were fine until upper elementary or junior high and then began to struggle academically and/or socially. Many of the children had to repeat a grade, which presents another challenge to deal with in itself. If, as a parent, you could avoid all those issues by waiting now, why wouldn't you?!
We opted to wait another year to send our son to K. We are SO GLAD we did. Our son excels academically and socially and he is a leader in his class. He is able to stand up for what he believes and what is right. He has the maturity to make wise choices. I believe waiting to send him to K has given him an advantage and has made school easier and much more fun. I would much prefer he be a leader than to be a follower of a leader who may or may not be a good influence.
It is not a negative thing to wait to send your son on to K. The way we approached it with our son was that we were "redshirting" him. We told him he was not required to go to K at age 5 and parents were able to choose what was best for their child. We explained to him that waiting was what was best for him because he would then be one of the oldest in his class and school would probably be easier. We NEVER used the term "held back". We opted to send him to a private Pre-K program that met 5 half-days a week and when we sent him to K the next fall he walked in confidently and never turned to look back!
I say wait to send him....it will only benefit him!