Brit - Let me start off by saying that only you know what is right for your child. I have volunteered in countless kindergarten classes, and there are always a few kids (boys and girls) who are a little less mature, especially at the beginning of the year. Most of them, however, outgrow it and catch up to the rest by the end of the year. If you don't feel he is ready, it is your right to hold him back. If however, you feel he is ready academically, I think he will be fine to go ahead and go. I was a very small child (33lbs 39 inches tall) at had an August birthday when I entered kindergarten. I never regretted being the youngest or smallest.
To many of the previous posters, I am always surprised at how eager people are to suggest holding a child back a year. It is no wonder that we are falling behind the rest of the world academically. In America, we no longer instill the drive for academics in our children. It is much more important to "us" that our children are fulfilled socially and athletically in school rather than academically. While I believe that the social aspect of school is important, it is not THE MOST important part of school. The focus really should be on academics.
If you have taught your child how to handle the social pitfalls of school--and all kids will have them whether they are the cutest, smallest, tallest, biggest, slowest, whatever--then they will be able to handle whatever social difficulties come their way, provided you are there for them and help them through it.
We have been in the opposite boat with two of our children. Two of our five kids have October birthdays, and hence, completely miss the September 1st deadline set by the state of Utah. We ended up having to skip our son a grade later on--he was completely bored in the current grade that he was in. It was HARD skipping a grade, much harder than if he could have just started kindergarten as a "young" kindergartner. In the long run, however, it has been the right thing to do as he was becoming increasingly disillusioned by the monotony of school that was far too easy for him.
Now, we again have a little one that won't turn five until October. She is completely ready for kindergarten NOW, and will be even more ready this fall. This coming year, I am going to either put her in a private kindergarten or do home schooling of kindergarten in addition to sending her to preschool (for the THIRD year in a row). Then, the following year, rather than enrolling her in kindergarten as one of the oldest, she will go into 1st grade as the youngest.
By the way, ALL of my kids are really little--only 10th to 15th percentile on the growth charts, so they are small whether they are the oldest or youngest in the class. We have taught them how it doesn't matter. None of them feel like it is a problem at all. My son cares the very least about it. I don't buy into the excuse that your child being small will cause problems. It is up the parents to teach a child that size is irrelevant. Teach them how to cope with being different--whatever their being different is.
Brit--good luck with your decision.