The situation will not be solved until all contact with the scammer is broken completely.
What may help is to get his psychiatrist, social worker, others responsible for his care involved.
It would help to remove his cell phone and any PC and stop his access to them for good. The internet is not a good place for someone with mental disabilities.
Possible involuntary commitment to a mental health facility to break the scammer's brainwashing of him might be needed, and will take strength by the family to do it. But when he is hurting himself by sending money he needs to a man impersonating a woman and does not realize it nor accept it, it might be necessary to save him from more hurt.
He has been put into a highly delusional and self-destructive state by the scammer and he needs professional help at this point.