Start with something that is fun like this (Score 1)

How I got into programming was probably the easiest and best way for a kid to do it. It started off with my favorite PC game called Duke Nukem 3D and there was a simple map editor. Although it resembled more among the lines of autocad; there were still objects with parameters that you set but it was all relatively simple and object oriented with minor event driven programming. It definitely would be a great start because it would allow him to start in an area where he enjoys it the most; and its not strictly text base coding like most people are suggesting; Kids won't want to do that. However, finding something similar to that now a days may also be a little more difficult; I'm sure if you look around you can find something similar. Have him start creating his own games or maps that he can play in (also try maybe making a flash based game with him) those are easy and won't take long. See if further down the line his school has a technology preparation course or special computer courses that he could attend if he feels he likes it. And keep in mind he's a kid and keep it fun!