If he doesn't have private insurance he will need to stay in the VA system, which means treatment by VA doctors or possible referral to outside doctors. Under the Mission Act that JNF mentions above he might qualify for medical care outside the VA if he needs expertise not available at his local VA hospital or possibly if he lives too far away from a VA hospital. I've posted a link below to Mission Act eligibility criteria.
I have VA medical coverage but get non-VA treatment for my prostate cancer at Dana-Farber covered by my Blue-Cross Medicare Advantage plan. Although the VA does not currently treat my PCa my VA primary care doctor says if I should need expensive medications that are not fully covered by my Blue Cross plan she will refer me to a VA oncologist. My Dana-Farber oncologist says they (D-F) have worked in collaboration with the VA for other patients.
Like Steve, I am also a Vietnam vet so any VA care for me is fully covered because of presumed Agent Orange exposure. https://www.blogs.va.gov/vantage/58621/new-eligibility-criteria-a-major-improvement-over-existing-rules/#:~:text=veteran%20was%20eligible%20under%20the%2040-mile%20criterion%20under,location%20that%20would%20qualify%20them%20under%20that%20criterion.