Hi. First of all, please accept my virtual hug. I have two sons, one at uni, while the other has just moved from a small school to a large community college for A levels. My first son was born with an innate ability to socialise. My youngest, on the other hand, fits the definition of introversion. It's been quite the journey to support two very different characters into (almost) adulthood!
I read your entire first post to my younger son who immediately recognised his own characteristics. He asks that I pass on some key moments of change that have had impact for him. He feels very strongly that when I gave him more responsibility, he rose to meet it. For us, that happened two years ago when I divorced and my eldest left for uni. My younger son was suddenly the man of the house, and I think that he quite liked helping solve often very practical problems for/with me. For example, I have arthritis, so my son really helped with things like moving house/organising the attic/wrangling power tools. He truly didn't 'do' very much, but on reflection he feels that solving problems was empowering.
Money was short, so he got a weekend job stacking shelves in a shop. He persevered, even though 'people' do his head in, because cash is a necessity! He lost his first job in a restaurant, and I can see why -- too much interaction with LOTS of different people was pretty tough. Stacking shelves on the other hand meant he could be helpful to the public, so stretch those social skills, without the pressure of 'serving' them, IYKWIM. Losing the first job was a real knock, but he survived. He's loved earning his own cash via the second job, and buying his own clothes etc, which has helped him fit in at college too.
My son suggests that your son be given real responsibility, where possible, for the normal everyday things that any functioning adult should manage -- no home hair cuts, or you choosing his shoes, for example. My son prefers to save up hated social chores and simply attack them one morning a month. That way he can emotionally gird his loins. LOL. He also says that school were very helpful. They recognised he was better at doing than socialising, and set him up for success by giving him responsibility behind the scenes. That's the kind of help that takes time, so perhaps talk to the teacher who expressed (somewhat thoughtless) concern. Perhaps they can work on an actual strategy with your boy rather than just voicing dire warnings!
Most of all, my son wants to pass on that he's far from typically manly, but he has a lovely girlfriend now who appreciates his quiet and gentle charm. University still worries me for my quiet boy, but not quite so much lately, and I truly hope you can get to a similar place too. 