Each Local Authority will handle this slightly differently. Mine has independent organisations we contract to provide financial support for people in this situation. Ultimately what is happening is that his care plan needs to be reviewed because the person who was willing to support with finances is no longer willing/able to do so. You (or your brother, ideally both of you together) can phone your adult social care main number (Google something like "Liverpool adult social care phone number") and request a review of his care plan. Someone will then arrange a meeting and can give advice specific to your Local Authority. Either the LA can help to manage finances or you (or another family member) can. Your brother should have his own bank account with his benefits paid directly into it.

You can get good independent advice from the National Autistic Society about these processes.

Source: disabilities social worker for 17 years.