Indeed there is no known footage of him as an adult. There are footages of him as a baby like on this video :
http://www.youtube.com/watch?v=S40DdlD9JxI
It's surely because he made a very little amount of live concerts because he didn't feel confident enough on stage, he played a quiet, sweet and acoustic folk at a time where rock N'roll was the fashionably movement, young people of his generation listened in majority to Hendrix, Joplin, The doors, The stones etc... And it seems that most of the people in the audience weren't very attentive to this shy quiet acoustic folk singer that spent a lot of time to tune his guitar between the songs. And it assaulted him, this lack of attention. It disgusted him of the stage. He wasn't commercial at all. He only gave one single interview and the reporter that interviewed him told that he mumbled more than he spoke, made monosyllabic responses and never looked in his eyes of the interview, not even one time, he looked on the floor. And this involuntarily anti-commercial attitude is part of the reasons why he hadn't more succes by his lifetime, and that he fell in this depression, and maybe why he died.
But I found a very short and soundless video on youtube, where you can see a man from behind, walking in a festival in the 70's, and he strangely looks like Nick, same clothes, same posture, same size (very tall), so there are big chances it's not him, but it looks like him :
http://www.youtube.com/watch?v=CZFnCdTCSXw
