ESA to UC appointees bank account

Hi ,
I am my son's appointee and received the ESA to UC migration letter.
My first question is that his ESA and PIP is paid into my joint account with my husband and has been since my son was a baby.
I am concerned about the "how much is in this account" section of the form....it isn't his account he lacks mental capacity and isn't allowed a bank account, I receive carers allowance but the bank balance isn't just his money. He has no savings. The form seems to assume this is his personal money. it is under £6000 but wrong to think it is only his money how do I deal with this question?

Many thanks in advance

Hi Thank you for your reply,
My son does not have mental capacity and is legally not allowed to open a bank account, I have been through this with the bank RE: his Direct Payments for Social Care

Thank you for your advice, I am also concerned that my son has no photo id or acceptable ID as he lives with his family so bills etc are not in his name. He couldn't go to the job centre due to his disability so I hope a home visit is more understanding that someone with complex needs doesn't always fit their criteria.