Bank closing bank account out of the blue

I think sometimes banks close accounts if there hasn’t been any movement for a period of time. It’s better for you if him to contact them quickly or pop some money in. Otherwise it can take a time to get access to these accounts.
M &S have done it to us a few times. We pop another £20 in and all is well.

His bank address is our address so I have opened any letters and shared content with him. He hasn't had an income for 9 months having saved up for this big trip, now back at work. He still has a positive balance though.

Santander did announce they were closing certain accounts but were inviting customers to change to a new account, I think.
If that is the case, did your son miss the letters if he was travelling?
Or has he not paid money into his account regularly and been using up the balance when travelling?

Banks are making arbitrary decisions lately, Nationwide being another one.