This is a serious situation and standing up to the people threatening him is not a viable option given his physical condition. That kind of confrontation ends badly for someone who can barely walk, and the people doing the extorting know exactly what they are doing by targeting someone vulnerable.
The most immediate protective measure available to him is requesting protective custody. He can make that request directly to any staff member or officer, and the facility is obligated to take it seriously. Protective custody means he gets separated from the general population and housed in a more controlled environment where the people threatening him cannot reach him.
The significant downside is that protective custody is essentially solitary confinement. The same restrictions that apply to disciplinary segregation apply here, limited movement, reduced privileges, and isolation. For a 67-year-old man in poor health that environment carries its own hardships. But it is considerably safer than remaining in the general population with active threats against him.
Alongside the protective custody request, he needs to file a formal written grievance documenting the extortion and the threats with as much specific detail as possible, names if he knows them, dates, what was demanded, and what was threatened. That creates an official record that the facility is aware of a safety issue and is legally obligated to address. Copy everything and have him get that paperwork to you so there is a record on the outside as well.
From your end, call the facility and ask to speak with the warden's office. Put your concerns in writing and send it certified mail. A paper trail that exists on both sides is the strongest protection available to him right now.