You can do a Voice of customer style interview. And do it in person/on phone. Do not read it though like you are interrogating him. Keep it conversational but have a bank of questions to get answers.

You can ask things like. “Have you tried offering a Ride?” (Yes you already know the answer but they might offer some tid bits here). If you have what did you think of the process? If Not was there a reason you didn’t put in a Ride? What keeps you from offering rides on your daily routine? Thinking about the next trip your are making what could the app do to help make it easier to offer up a ride.

Make sure your questions are open ended not yes no not 1-5, But conversational. Also make sure the question would be answered and thus giving you actionable data. Like if I said No I have tried, and said the app is just difficult. you still have no idea why, is it the app, the idea of setting up the route, the interface, pricing etc. So Its hard doesn’t tell you anything that you can fix.