The Animorphs series was a big hit with my son at that age (actually he's just re reading them now at 8). They have violence but my sensitive son is fine with them. Not sure that they're read in the US so much but Enid Blyton's Famous Five and Secret Island books were also a hit at six. Simply written but my son loved the premise that 4 kids, only slightly older than himself, just go off for days and have adventures. Captain Underpants and Andy Griffith's 13 Story Treehouse for laughs along with Asterix. He also did The Hobbit and the first of the LOTR as an older 6 along with Jules Verne. It's a pretty eclectic mix. Never quite sure what will capture him.