One option is to fictionalize your narrative, such as...
Over the last six months, I've interviewed some of the people who were involved in the young man's life. In reviewing their contributions, I like to imagine them all sitting together in a room and discussing their insights.
Mary is the subject's older sister, an advertising executive living in New York. "He was always a shy child. I remember trying to include him in social events, but he almost always stayed home."
Bill is quick to contradict his sister. "I wasn't shy. I just didn't like your friends."
I can almost hear the siblings' discourse even though, of course, I met with each of them separately. My imaginary meeting really gets interesting when Bill's mother, Susan enters the room.
By creating a fictional encounter, you get to leverage the rich tool set of "He said" and "She said" alternatives to craft an information rich narrative, which accurately integrates your non-fiction quotes, with fictional (but fact based) responses from Bill (the subject) and yourself (the narrator).