I checked out Lisinopril, and it's an ACE inhibitor. If that works for your father, here are some alternatives from Dr. Stephen Sinatra.
Fortunately, there are natural alternatives to ACE inhibitors for treating high blood pressure levels. These include:
•Hawthorn berry (500 mg two to three times per day) increases blood flow in smaller vessels, acting much like ACE inhibitors, as it decreases blood pressure.
•Garlic (500–1,000 mg daily)
•Magnesium (400 mg daily)
•CoQ10 (100–300 mg daily in divided doses)
•Fish oil (2–4 grams per day)
•L-arginine (2–3 grams three times per day
*Note -- Dr. Sinatra listed calcium also, but I took it out.