My expert on these Medicare coverage issues is Diane Archer. You (and others with Medicare coverage issues) can reach out to her at: Diane Archer
I sent her your question and here is her answer:
I am assuming her son is on Medicare as a result of a disability. The issue is likely that he cannot get Medigap coverage in Wisconsin. In most states, people under 65 with Medicare have no guaranteed right to Medigap. He could enroll in traditional Medicare but his out of pocket costs could be considerable.
I wonder whether her son is eligible for Medicaid or a Medicare Savings Program. If so, that could allow him to be in traditional Medicare with financial protection.