Logo.  It's the gold-standard of introductory programming languages.  It has also been described as "lisp without the parenthesis", so it should have many of the lisp fans on here on board.<p>Finally, it's just fun.  I have fond memories of moving that little turtle around with simple commands like "FORWARD 100" and "RIGHT 90", when I was introduced to programming at around the same age as your nephew.  You really couldn't ask for a better first language.