I don't think public transportation would be a real option, because that would limit it to Greyhound, and taking a trip by bus takes planning, and he would probably have to stay in a motel overnight for the return trip - the money spent on it could just as easily be spent on car maintainance, which would be better in the long term.
Aside from all the other teenage jobs mentioned, look for labor jobs, like mowing lawns or garden maintainance. You will work your ass off, but it will probably pay a little better than minimum wage, and you get to enjoy the summer weather. I did that one summer in college, and worked at a country club another summer.
This thread makes me realize how great I had it as a kid. I went to my first concert my senior year of high school, and drove 3 friends to it as well, and never had any problems with the 'rents.