It is wonderful there is even one child he speaks to in school! It's too bad that kid is a follower hanger-on type and not a real peer perhaps, but it's something. And your DS sounds like he has a good social relationship with you, that is great.
Nothing is going to "balance" your son out. Of course no one wants him to be lonely but that is not always something we can just fix. Not having good friends is not necessarily something that can be controlled by putting him around more and more kids who's favorite activity is chasing eachother around yelling (most 7 year old boys), if that's not his idea of fun.
Isolating him socially at home isn't ideal either but as he's been to school some and it's not working out, currently homeschooling may be the best option.
I have a very gifted brother who was beat up, left out, always socially not it in elementary school. He was not terribly happy but kept going because no one offered a different option, came home and cried, etc. From 10-14 he was sent to a gifted school over an hour from home, and liked that much better, still I don't think made many real friends. In public high school he never ever once called anyone on the phone from home that I'm aware of, though once every now and then another boy would show up to play cards or chess with him so he must have known some kids from school (though much of it was likely initiated by my other brother). By late high school there were a couple guys he hung out with, ate lunch with, studied with, I'm pretty sure he thought of them as friends but they weren't constant companions. Dating was just not even a thought in high school. He was odd, my mother nagged him constantly to wash and put on deodorant and he sometimes did.
It was really college where he met a bunch of guys he became good friends with, they appreciated his humor and he appreciated theirs. They did harmless geeky college prank things together. And a year or so later his wife met him, asked him out, initiated the first kiss, etc. He's very happily married, he's a computer geek, he's also a wonderful guy who's social and sweet and compassionate, he's just not really your average guy. He's just who he is and his family was nice to him and supportive and were his friends until he found others. He basically had to wait until his age-mates grew up before he could really have something to talk about with them. Or maybe he just didn't meet the right kid earlier, I don't know, college certainly concentrated the geeky smart guys for him.
Teachers all want to have an effect and yes perhaps your son's current social attitude is self centered. But more disclipline isn't necessarily going to change him. Perhaps he's just who he is and will find friends when they come along, it may not be for years even if you do go to robotics classes or whatnot. If he is empathetic and socially capable with you then he can be towards others and will likely find good friends eventually.
If he's sometimes manipulative and demands to be the center of attention with you on the other hand (which my brother did not), then you can probably work on that. You may already do it but can demand he respect your ideas and interests in a fair way (such as you getting to choose what game you play half the time, choose the color of gamepiece to be half the time, the restaurant, who gets to talk, etc), act hurt if he says or does anything hurtful or tries to manipulate you. Ie you can act like a respectful but independent minded (non-indulgent-parent) human would and make sure your friendship with him is model of a real true friendship, so that later he is truly able to have one with another person.
Polly