get a referral from his primary care physician.
Explanation: An HMO is an acronym for Health Maintenance Organisation. They are intermediaries/middle men for companies and health services companies(dentist, optician, hospitals etc). Companies pay HMOs to cover the health insurance of their employees and families of their employees.
Harry has to get a referral from his primary care physician because the diagnosis has to come from them in order for his HMO to approve his visit to the podiatrist.