My advice, as someone who is similar to how you describe your son - is:
1, try lots of new things. For people who find it hard to make friends, it can often take meeting a lot of people for friendship to click.
The truth is that often quieter/shy people need to find a confident person to latch on to. It sounds like your son had that - but it didn't work. My own experience that of my quiet friends, is that as teenagers, you often see a pairing of quit kids with louder and confident kids. When it works well, it is mutually beneficial. The quiet kid, needs the confident kid to actually be the proactive one, initiating the friendship, making plans, affirming the relationship etc.
But often the confident kid gets a lot of it too, especially the reliability and stability that the quiet child provides.
Point is, as a quiet person to find that person who clicks with you, you need to meet a lot of people. Social and confident people can walk into any group and probably make friends. Shy and quiet and socially awkward people can't do that. So even though it is hard, what they actually need is a variety of different environments, to give them the opportunity of finding one person or group that clicks.
If it were me, I would encourage new groups, specifically focused on structured activities (easier for socially awkward people), and related to his existing interests.
DofE? Ten Tours? both contain structured Hiking, one is military linked..
Scouts/outdoor groups, similar to cadets?
Volunteer groups that do activities to help the community?
Wood working, making/building groups = the home for generations of social awkward guys
Bird watching? Photography groups? Model building.. model boats? Warhammer? All good hobbies for boys who don't like crazy social environments.
Have you tried a boardgame group? Amazing for people who have a hard time socially, because after you get pass the fear of joining the group, all of the social interaction happens around structured games. Way easier.