OP, this is going to sounds strange, but you're taking this too personally, instead of taking it on the chin as valuable feedback about a gap in your son's learning that you can help him address. He clearly doesn't know how to get on in the world of work, which is fine, as he's only 18. But did he never learn public speaking or debating in school, or have a part time job or is he (and you) just hoping he can get by never stepping outside his comfort zone? If so, you are doing him a massive disservice by allowing him to think like this.
Unfortunately, in the UK, people don't like silence and they are suspicious/feel uncomfortable around those that never speak or make an effort to enage with others. It makes them feel ill at ease. This may very well be their problem, but this is the world your son lives in. He needs to practice. He needs to be taught how to engage with adults/peers/those that are senior to him (and understand why it is important). Greet people with a smile and a 'good morning'. Ask if anyone needs help before the end of the day and say goodbye before you leave. Ask how someone's weekend has been in the break room or when working in close proximity. If the people next to him are talking about football and try to engage him, he should participate - even if he doesn't like football, say he's not into it but loves tennis or say, actually, he's not into sport but he's building his own laptop or is into film and just saw the latest x (whatever the film is).
The above doesn't have to mean he's chatting all the time or pretending to be the life and soul of the party. He just has to have basic conversational and social skills, which can be taught. Of course he doesn't have to ask questions if he doesn't have any (why would a junior not have questions though? Is he only doing the bare minimum? Is he too smart for what he's doing? If the latter, he should take an interest and ask something relevant - what's the next step? That's interesting, what sort of clients buy that? How did you get started here? etc), but no one wants to work with someone that can be mistaken for sullen or meek. They want someone they can work comfortably beside every day, who will speak up if they are hurt/in danger/notice a mistake/aren't sure of something/have a laugh at a joke or a 2min chat about the weekend.
Some tough love - you and your son need to toughen up. This is work. It's ok he got it wrong this time. He's young and will make loads more mistakes (as we all do). But the answer is not to hide under his metaphorical bed and give up. Accept he didn't behave in the way expected, think about whether there was an element of truth and, if there was, plan how he can behave differently next time. (He can even use it in an interview in terms of questions about difficult situations and what he's learned, how he would adapt next time.) It's not the huge deal it feels right now. He tried one job; it wasn't a good fit. That will happen a lot more in life. He just needs to keep going.