Although you were out of order for gesturing and mouthing off, do you not feel that he exacerbated the situation by driving close enough to see what radio station you were tuned into?
You have tried to apologize and he wasn't big enough to accept. Road ragers seldom back down, and some have been known to take things to extremes, so my advice would be to leave it alone now, learn from the experience and move on.
John