Love the quilt!! My little guy used to be a cars fan and he would have loved that. Personally, I like my binding to have a little pizzaz. I would go for red! I would do something fun, like red dots, or stripes with red/blue/black that incorporates all the colors. If you go with black, which would look nice, but not show up much, I'd probably go for a black dot fabric, like maybe black w/red dots, or something like that.
Have fun, and I hope your son loves his quilt. I just finished my son's baseball quilt and he refuses to sleep without "mommy's love" on top of him!!