Why not just ignore him? Flipping people off gets you no where.
Are you sure he was racing you, maybe he didn't even step on the gas?
Granted, if he hadn't, you woulda been a lot further ahead...hmm
But yea, I understand where you're coming from, but maybe it'd be more worthwhile to not do anything. You probably did exactly what he wanted you to do, get pissed off about nothing. I've had people flip me off, and I just wave with a smile and laugh. It doesn't bother me one bit.
Nice kill, none the less.