I think his upcoming job may well dramatically change things for him, even if only temporary. He will meet new people, colleagues, customers, suppliers, etc and that will build his confidence (I hope it's not a remote WFH desk/PC based job!!).
He sounds like I was as I had no real friends at school, just passing acquaintances, and never mixed with other pupils outside school for the entire time I was at secondary. My son was the same. My DH was the same!
For all three of us, in different places/different ways/different ages, it was getting out into the real working world that changed us, gave us new experiences, meet new people, change our outlook, etc. It led to both DH and I doing voluntary work alongside a full time job (again, independently, different years etc). It was one of the voluntary roles where we met - neither of us "dated" nor socialised, so we only met people through work or the voluntary work.
Son is slightly different as he hasn't done voluntary work, but did go to Uni (awful experience due to covid years), then had to move away and live independently for his first graduate job, but it was under the graduate job that he got friendly with a couple of sandwich year work experience lads (his first real in person friends), they returned to uni for a year, but he moved in with them a year later in a new city where all three of them found work. Now he has two very good friends who he spends all his time with, living with them, socialising with them etc.
So for the three of us, it was work that made the difference, gave us confidence, improved our chances of meeting new friends, etc.