One other thought: is your DS an introvert or an extrovert? (And please note that I'm not asking if he's shy--shy and introverted are two different things.) Does he prefer time alone to recharge, even if he likes to be social? Does being social make him tired, even cranky? Or does he get energized by being with people?
Are you an extrovert? Do you crave time with people? Your posts make it sound to me like you might be an extrovert. That's important, too.
If your son is an introvert, he may just prefer to play alone a lot, and that's fine as long as it's fine with him. Ultimately, the big deal with friends is going from 0 to 1. If he's got one playmate that he sees semi-regularly and if he's happy, then he's probably not a kid you need to worry about.

K-