It's not an insurmountable challenge, though. We are heavily involved in Cub Scouts, and he is into sports. He's taking Tae Kwan Do, golf and tennis at the moment. Those are good opportunities to reinforce discipline, even though some of the kids have behavior problems. We tell our son to pay attention to the coach, and not to the "problem children" who are in his classes. We make a point of using the unruly children as examples of bad behavior bringing bad consequences...nearly every tennis class includes one particularly mouthy kid being put on the bench while the others continue to play.
We're fortunate in that there are children his age in our neighborhood, some in private Christian/Catholic schools, and some in public schools. We make a point of knowing their parents, and they are good families. Most parents are receptive to having their children become playmates with kids the right parents.
Lord, this post has dragged on. But my point is that I believe social interaction with other children is important, and, with a little effort, homeschool parents can find opportunities.
Just like with public schooling, the parents MUST be involved with their childrens' education and lives.