While it seems shocking that the dad made no provision for his disabled son, it would be helpful if someone with a legal background could tell the op if anything can be done. Depending on the disability, maybe the father didn’t expect his son to outlive him?
It shouldn’t be too difficult to get future disability allowance paid to the disabled son, as it is his. I’m sure there will be a bit of red tape about this, especially if a bank account has to be opened.
Does the son attend a day service? There might be a social worker there who can help. If he attends a day service, is there a residential service there too? Unfortunately, it is not unusual for a disabled person to need residential care after a carer parent dies.
Is there a family member who is advocating for the disabled son? It sounds like his brother is part of the problem, not part of the solution. Is there a possibility of him living with another family member?
On top of all this, depending on his intellectual abilities, the disabled son is probably either very bewildered by the absence of his father, or grieving. He is likely to need psychological support.