You can't control friendships. Just because there are kids in the neighborhood doesn't mean they have a lot in common. As you have noted, there is a large age difference among all the boys. Your son is squarely in the middle, and you must acknowledge that there is a difference in maturity between 9 year olds and 12 year olds, just as there is between 15 year olds and 12 year olds.
I think you have to let friendships evolve and develop without getting involved.
I think it's incredibly foolhardy and outright rude to tell your child he is to inform friends who they are to play with and in what numbers. I think the message you are TRYING to convey is "I don't want my son, "Johnny", to be excluded or to be second choice." Instead, what you are teaching Johnny is "If Bobby isn't home, then Tommy has to sit home just like I do. If Tommy has a date with another friend, or is at the doctor's office or is at soccer practice, then neither Bobby nor I am allowed to play together." What is the purpose of this message? You are teaching kids that they are not allowed to play with their friends unless they are in a group that you dictate. This is rude, silly and bound to backfire on you as the "meddling and unreasonable parent" in the neighborhood.
Now, if you were to teach Johnny that, if Bobby comes over, to suggest that they also ring Tommy's doorbell, great. He can say, "It's so much fun when we all play together." But then you back away! If 2 kids have something more in common and want to play together, so what?? It might be different on a different day. Please allow kids to choose their activities and friends. You can also teach your son some social skills so that he is able to go over to Tommy and Billy and say, "Hey guys, what's up? Want to play catch?"
Give your son plenty of play dates with other friends from school so he isn't entirely dependent on these young boys to rule his social life, and encourage him to have friends in a variety of areas - school, sports, scouts, choral group, whatever. If your son needs social skills, work on that. But teaching him to say "no" to the kid ringing the doorbell reaching out to him is doing the exact opposite of what you want! It's putting up a barrier instead of permitting relationships to emerge and develop organically.
He's 12. It's time for him to be responsible for friendships and for you to just do the transportation and provide plenty of equipment and supplies so that yours is the fun house to visit. Sleds, bikes, a basketball hoop, some juice in the fridge and popsicles in the freezer - that's what kids need to feel welcome.