There are a number of drugs available for the treatment of pulmonary hypertension, the most widely used one being Bosentan, which is approved by the FDA.
Your husband should arrange to be seen at a Pulmonary Hypertension Clinic. Such clinics exist at most University Hospitals. That way you will get an expert opinion on which of the available drugs might be best for your husband and may be able to avoid the expense of Viagra.