Your little boy sounds like he is getting just what he needs and wants. Not everybody has to be a social butterfly. He may find more common ground with others as his interests develop. Probably he'll never be the life and soul of the party, but I know a number of children who were similar to your son at that age and made one or two good friends as they got older, and sometimes even found a group focused on an area they were really interested in.
But this "It's not just about his needs anyway - I know I'll get depressed if I have to spend the next 10 years in the house because he just wants to sit and take a computer apart and my 2 year old also needs social interaction" says to me that he may have to learn to put up with getting dragged out places he isn't terribly keen on, so you and his brother can get your social needs met.
That's part of being a family, and so long as he doesn't absolutely hate going out, I'm sure you can find a compromise. For example, perhaps he can bring a book or something to play with in case he isn't feeling sociable. At the home ed groups I go to, there are generally one or two kids sitting in the corner or waiting in the car doing their own thing while the rest of the family socialises. Nobody thinks anything of it. You could invite other families to your home and give your older son permission to go off and do his own thing. Just because you and your 2yo are having a playdate with another family doesn't mean he has to join in. So long as the other family has been told that he isn't always sociable, and aren't expecting him to join in, they won't be offended. If you have some interesting stuff for visiting children to play with, they will probably enjoy themselves regardless of whether your son plays with them.
And it's likely you won't feel you have to wait till he is 16 to go out without him! Many HE families I know, including my own, regularly leave older children home alone while parents take younger children out. I know of more HE children who are left alone from a younger age than schoolchildren, though of course every family and every child is different and I also know HE teens who are never left alone.
Leaving kids home alone isn't done out of necessity: older kids can generally be taken along if their parents feel they need supervision. In my opinion HE children, because they spend so much time with their parents, get more coaching in life skills and situations. And having spent so much time together in a range of situations, their parents actually know them better. As a result, parents feel that they know whether their kids are ready to be left alone, and can predict what the likeliest potential problems are and how they can prepare their children for those situations. I always used to be quite baffled when I read posts from parents (of schoolchildren) asking, "At what age should my child be allowed to...?" and I'd think, "Well, how on earth should I know? I don't even know her, or your situation. She's YOUR child; don't YOU know whether she can safely do that?" But then, talking to people, I eventually realised that many parents do not actually see their children for enough hours of the day to be able to predict how they will react. I think this may contribute to a culture of overprotectiveness, a better-safe-than-sorry, you-never-know-what-they-might-do attitude.
Anyway, your little boy sounds absolutely lovely!! I hope you are enjoying your time together and that you will find ways to get what you and your younger son need too!