Back dated council tax bill

Salbags

Hi ,I am new on here and joined in the hope someone could shed light on this situation please.
I am a full time carer with health issues of my own for which I am in receipt of carers allowance. I look after someone with mobility issues ,brain damage and mental health issues.
Out of the blue he has received back dated council tax bills back dated from 2014 through to 2017. 
He became a state pensioner in November of 2020 ,I am very confused as they said this has happened due a change in circumstances .
1. why only bill up to 2017 and not up November  2020 ?
2. Does he have grounds of appeal to get this bill written off under the mental health act on the grounds it is so far back ?
any advice would be welcome .

many thanks in advance

Savvy_Sue

I think you'll have to ask the council WHAT the change of circumstances is. 

Do you live with this person? 

Do they now have a disregard because of mental incapacity? 

Did they live alone, if not with whom were they living at the time billed for?