Aus - have you considered contacting media to see if anyone is interested in the story? If a journalist or paper feels they can get a half-decent story out of it they can be motivated to address the bank publicly, which can pressure them to take responsibility (understanding that responsibility here is a grey area). The story has been done before, but it's one that requires regularly repeating in the interests of public awareness. Hope your boy gets his cash back and invests in sandwiches instead.
I have nothing to add really, but did the bank know he was vulnerable? And are there accounts designed for people that could be more vulnerable to fraud, with increased vigilance on transactions?
Sorry that you and your son are going through this. I often stop and ask myself - how can I be sure that this is not a scam? I think maybe a lot of us are more vulnerable than we imagine 🙁
Thanks for the encouragement. The bank have closed the case from their perspective so I'm prepping for the Ombudsman.