Three things:
1. Sounds like you're doing a lot right; your son sounds like a terrific kid.
2. I could use some parenting advice of my own, so I feel unworthy offering it to others. But, since you asked....
3. I would tell my son that "the finger" is a really dangerous gesture. It's practically "fighting words" to some people, and you can't go around flipping it unless you're ready and able to physically defend yourself. I've lost my temper before (I yell at drivers, I admit) but I always keep my finger in check...I honestly think it's a dangerous thing to toss around in public, especially in traffic.