One thing I think you should keep in mind is that people tend to label ANY child who enjoys his own company as "socially behind." From what you've said, he doesn't totally shut others out when they want to join him -- he just enjoys what he's doing enough to do it alone if no one joins him.
I was a lot like this: I loved imaginary play and it was great if others joined me, but I'd do it on my own if no one else was interested. The problem was, my teachers (by second grade) saw this as a problem at recess, and insisted I had to do "playground" stuff. My oldest is a lot like me. She enjoys other kids, but will do her own thing if others aren't playing something she'd like to play.
I commented to a friend that dd seems to enjoy playing with other kids once or twice a week, but gets grouchy if we have friends over too much 'cause she's used to having her own space a lot of the time. My friend said, "GROWING hurts and can make kids grouchy." She seemed to feel, like many others, that I should just make her get used to more frequent social contact rather than honouring her unique personality and preferences.
I think the best help we can give our kids is to affirm their uniqueness and recognize the ability to enjoy solitude is a GOOD thing. If your child's actually unhappy playing alone you can certainly encourage him to reach out to others, but from what you're saying it sounds more like he's quite happy and simply feels no need for others to join him -- it's okay if they do, okay if they don't. I wouldn't mess with a good thing.