Changing address on a bank when someone is unable to verbally communicate

Hi I'm looking for some advice, I care for a young man age 19. He' unable to verbally communicate and has little use of his hands. His mum is trying to change the address on his bank account, but as the young man can' sign himself they said it can't be done. As you can imagine this is frustrating for his mum, they have said she needs to apply to the court for power of attorney. Now I'm sure this again is something the young man would have to agree to, and this is not possible. Can someone advice if they have had the same problem, and how you managed a way round it. They won't even let mum get any money out for the young man.
Thanks.