I think you are referring to the "War" album cover art. His name is Peter Rowen. You can read about it here: http://news.yahoo.com/blogs/the-difference/boy-behind-u2-album-covers-shares-story-behind-221348041.html. read more
Peter Rowen, who appeared on the covers of U2 albums"Boy" (1980) and"War" (1983) as a child, is all grown up and a photographer and father living in Dublin. And he is still close with Bono and The Edge. Two years ago, Rowen, now 37, photographed the band's Dublin concert, in addition to others. read more