While we are not so much worried for the K years - we do worry about the impact of being the smaller/youngest one when he is age 12-13 when according to research is when some kids are more affected by their age/grade level.
I find this assessment to be funny because I work with middle schoolers and 12-14 year olds are all over the map in terms of height, the degree to which they have gone through puberty, and maturity, especially physically. It's hard to say where your kid will be, but it's not necessarily true that the youngest is the smallest/most immature; there is a whole 2-3 year span that is normal. There are some 8th graders/freshmen who look like men, and others who look like little boys! Oh what an awkward time.
But as to the original question, I don't have a huge amount of personal experience because I am a May birthday myself (so on the younger end of the middle), but in general these things seem to be a lot more personality based than age based. By the time you get to college (or, according to Malcolm Gladwell, with regard to academics, 8th grade) these things tend to even out. I know one of the laziest guys I dated from high school was "redshirted" with an August birthday, and my best friend has a "non-redshirted" fall birthday and is incredibly self-directed. And there is everything in-between.