WOW! You deserve a lot of credit for not only being a single mom, working full time outside the home, etc. but, for also having the insight and wisdom to look at the situation from the perspective of your own sons "friend choosing method" ilo the much easier (common) response of blaming all those other terribly behaved kids! = )
I also think your concern is quite justified. Isn't it crazy to realize just how many of us still quite vividly remember our own friendship sagas from back when we were at or around your son's age?! Apparently these situations must of had some kind of effect on who we are and/or how we look at certain things today...
From a similar experience of my own youth, I recall my own Mother's response to the situation...which was basically a lesson of compassion. (...a really great virtue, growing more valuable to me with each year that passes in the lives of the 2 girls of my own...)
Back then, we (my Mom and I) had little laid back "chats" while participating in something we both enjoyed about the goings ons amidst my current age group, my views on friendships, etc. I'm now quite certain that her real aim/goal was to (a bit sneakily) get me to open up/share lots of dirt regarding the current friendship dramas of my own.
Once this background info. was gained "Mom" would move into gaining info. (in particular) on whom I thought might be the most "friendless" in my class. Afterwhich, she would then set to work making plans to setup not just one, but a series of "playdates" with the this very "one with least amount of friends" and of course myself.
In a nutshell, if we both went to the same school as children and you actually were without a ton of friends, I imagine (due to my Mother) we ended up hanging out quite a bit! = )
Skipping parts, obviously, of the whole "lesson" experience, the end results were that...yes, I learned compassion and actually ended up going from the very leader of the "super exclusive" group at my elementary school to the (at first a bit brashly and annoyingly) kid always trying to seek out and befriend all of the "lonely kids" on the playground.
Of course, kids should be kids and I'm not promoting too "heavy" a lesson per their age, however, empathy, compassion, insight and wisdom...well, who doesn't want their little angels to encompass that?! = )
Side Note...I imagine he'll be just fine whatever you choose to do, it sounds like he has a really good Mom on his side. = )