Both of my kids are April babies and even though my husband and I made the decision to home school them for a couple of years, we still waited until 6 to start them in kindergarten. And both of my kids learned to read when they were 4.(My son will be 8 in two weeks and is finishing up first grade; my daughter turned 6 a week ago and will start kindergarten this fall.) Neither my husband nor I went to kindergarten at all, but we both started first grade at 7 and both did well academically. (He was home schooled for several years, but I was not.)
However, I have a friend who is a teacher and told me there is a big jump between second grade and third grade academically and she said the extra year makes a huge difference in how well kids make that jump. She said that knowing nothing about the kids, within a week of school starting, she can usually tell you which third-graders started kindergarten at 5 and which ones started at 6.
As far as socialization goes, there are plenty of ways to get that -- Sunday School attendance and Vacation Bible Schools in the summer if you are church-goers, most libraries have storytime groups you can attend, any kind of "group" lessons -- swimming, gymnastics, ballet, karate, whatever your son might have an interest in -- even just playdates with other kids his age. (Preschool might also be a good option.) It'll come whenever he does start school, anyway, and having an additional year to mature might make a big difference in how he handles himself with other children.