Power of Attorney advice

jinnie29

Hi,
I'm asking for advice for my dad who is the sole carer of my brother, who is 49 yrs old, and has Down's Syndrome. My brother does not have capacity to manage his finances in any shape or form. About 8 yrs ago before my mum passed away, my parents set up an ISA for my brother in his name. My dad asked at the bank about moving the money from my brother's ISA into my brother's savings account. The bank told him he can't do this as he needs power of attorney. My brother has never had, nor will have, the capacity to manage his financial or welfare needs. He can't read and can only sign his first name. 
All correspondence from the benefits department are always addressed to my dad, on behalf of my brother, and do state that he is recognised as the person dealing with my brother's benefits.
From what I've read, power of attorney can't be achieved because my brother doesn't have capacity to grant it to my dad. Does it not matter that my dad has managed his affairs for all these years?
Any advice is appreciated.