We are not friends but guy sent request and I don't want to insult him?

This guy was senior to me and always topper so I know him. But I hardly remember(maybe once) talking to him in school. He lives in my city now and have sent me friend request on facebook. We were not really friends (we in 35-40 age group). By refusing tobe friend I don;t want to insult him. He probably sent friend request because we have 3 mutual friends on facebook. I am in dilema what to do? Thank you