I don't know if they still offer it, or if the requirements have changed, but Lloyds did have a "new to the UK" account where they were more lenient on address requirements. You did have to apply in a branch in person in the UK to open it and I think you still did have to be resident in the UK though. If I recall correctly, they were just more flexible as to what they accepted as proof of address since they recognised that being new, you wouldn't have official correspondence like a council tax bill.
With the new immigration rules and anti money-laundering measures, I'm not sure how lenient banks can be nowadays, though it's worth a try to see what's on offer. I *think* Lloyds and HSBC have international accounts that can be opened from the US in sterling, but I think those accounts are the types that command significant monthly fees and/or have high income requirements.
I suspect that to have a chance opening a UK account without an official UK address, he would need to inquire in person at individual branches. As a UK citizen, a branch *might* be a bit more flexible, but I'm not 100% sure.
Quite cruelly, returning UK citizens living abroad with their non-UK/EU families have been hit the hardest with the new immigration rules.