My 17 year old daughter is very much like your son. And I struggled for years the same as you are. She still has no close friends, by choice. (She is close to her siblings and cousins at least.) She just cannot connect with fellow teenagers.

As time passed, something magical happened... I realized that this is just who she is. I'm not worried anymore. She is now a Senior, focusing on her future, and finding happiness within herself.

Some people just move at a slower pace. The upside to my personal situation is that my daughter is very studious and focused on her goals. Her time will come when she opens up socially.. Your son's time will come too:) Just love and encourage him always. Also, therapy is always helpful.