UPDATE
Yesterday my son picked up a kid oriented graphic novel that we had read together and started reading it on his own! I posted this question to SE the day prior to that so I didn't have time to implement any of the ideas in this thread. So I guess maybe I was doing something right all along!
Regarding the school commute, I think I am going to mix things up a bit. I'm going to give him English language books that I know he enjoys (rather giving him books in his first language that I have no idea if he likes or not; I suppose that it is wrong to assume that a child should begin reading on his own using books in his native tongue). I'm also going to try playing educational English language podcasts in the car like Brains On! and Wow in the World. The rule was previously that the Leap Pad could only be played in the car for longish trips (maybe over 30 minutes). Maybe I'll extend that to one hour.