I will admit, I have been there and done that. Spent years of my youth stuck behind the screen gaming or programming. I eventually around 23 started getting out making friends and having a good time. It is not to late, if he is programming totally support it (I am sure you do) as if he can land a job doing something with it, that will help with his social skills. I did not get into programming professionally till I was 25 and I regret not pursuing it earlier. He could be 23 and easily making $80k a year if he sticks to it.. and with that kinda of money with no girlfriend or kids he can live a very social life then you will be coming back here "Help my son is out of control he is blowing money like it grows on trees and is out constantly partying!!", lol.
All jokes aside, I totally feel where you are coming from though, I am a "social" but anti-social kinda guy. I can survive in a crowd but my comfort is behind the keyboard. With these crazy times it may even be safer to have him stuck inside on the computer then out being social where all these crazy shootings are going on and idiots playing the "knockout" game.
I have a feeling if they had computers like this when I was a kid this post would have been from my dad as well. My best advise is especially if he is programming, support him, giving him some space and he may when ready hatch from the cocoon into something socially beautiful.
I also had a friend in high school who refused to leave home, refused to get a job doing anything a normal teen would do. He was very anti-social in a sense to the point I could only get him to hang out in his drive way. He would complain if I wanted to go somewhere, I would pay to take him to movies, and theme parks as he was my good friend and I wanted to get him out but he just wasn't into leaving the house.
Now, he is a graphics designer for 343 studio's and worked on halo 4, I am sure he is probably working on the newer one as well. He is very social, lives in Austin, TX has a bunch of friends I always see him posting things with them. He was a virgin until I'd say 25yr old if I remember. Unfortunately for him he found one, snatched it up and hasn't left since so he is missing out on all the wonderful fruit to try out there but he seems to live a happy life.
I get the concern, but speaking from experience.. I believe your son is going to be just fine. There are many benefits to not being exposed to the cruel world we live in.