For an 8-year-old? Get an inexpensive light that takes AAA batteries.
Why not wafer batteries? Because AAA are easier to pick up just about anywhere, and hard to mistake for the wrong one, and because they generally have a long running time.
Why not USB rechargeable? Because it's an 8-year-old. He'll forget to charge it. With AAAs he will be good to go for a few weeks or even a few months.
Why not a high-end light? Because he's not riding major roads at 10pm. He's tooling around the neighborhood and in bed by eight or nine (at least mine is).