I honestly do not understand our daughter. She has multiple medical issues, anxiety, and depression, and she isn't interested in any things that ordinary young 20s are (it was the same in high school. It took her 7 years to complete high school). She doesn't have friends, doesn't like shopping, mostly likes video games and sleeping and she deals with constant pain. I get it, somewhat. Life is really hard for her.
But some of our best times have been when I sit down next to her, pick up a video game controller, and ask her to assign me a car (or a character or whatever). I drive crazy, backwards, horribly (not on purpose, just old tired mother doing her best), and we end up laughing. I know that video games can be counter-productive, but for a solitary, lonely, struggling kid with medical or psychological or emotional issues, they can be a connection. Don't try to talk. Just ask "which pedal does what" or "what's the goal" or say "I'd like to hear that song that you love".
The other day she asked if we could place some bumper sticker on our car. It's from some video game (Halo? Mass Effect? no idea) but it's a realistic-looking political bumper sticker with the numbers '16 and then 2 names, like president and vice president, except that they're not real people, they're characters in this game. Anyway, I posted on FB a pic of our bumper sticker, just for laughs, and she posted a reply in public: "mom, I love that you try to understand me, even though I know you don't know who these people are, but they're important to me and I love that you let me put that bumper sticker on".
So, whatever your son likes - video games, putting together a computer, doing a puzzle, sculpting things out of clay, a music genre, a movie or sci-fi stuff, a band, whatever it is - even if you don't get it or like it - just sit with him and ask: "can you show me how to do this?" or "can I watch this with you?" . Don't let on that you think it's silly, or babyish, or too difficult, or impossible to understand. Simply meet him in his world. Appreciate something about it. "You're such a great car racer." "You really get how to put this together." "What an interesting character. Tell me about him." "What's the name of that band? Could I listen to something else that they've done?" You just might find a connection that will have meaning. It might not be what you expected. But with our kids who are challenged, by medical problems, by learning problems, by psychological problems, by emotional problems, by social problems, by physical problems, by perceived problems, by serious problems, the unexpected can be significant. And beautiful.