In my state there was a very noticeable reduction in road rage aggression when conceealed carry became common.
In your situation I'd incline to lock the brakes, at least if you're not at an angle where you're going to get flipped - he's already damaged your car, and you've got more power in your four brakes than in your engine. It'll also make a nice clear trail for the investigator, and avoid questions about one-sided aggression versus consensual fight. Also, if he's pushing a car with locked brakes, he may get sheet metal to cut one of his tires if you're lucky.
The next question is what happens if he incapacitates your vehicle and then comes after you. I used to keep a jack handle on the front floor, which was some comfort, but that's when a carry piece is really nice to have.
As some others have pointed out, you've described a scenario which makess a pretty good case for justified use of deadly force - but the real question is what response best serves your goals of survival, freedom, and minimizing damage (in that order). (If you had an armed passenger, it all would have been much simpler - good reason to pick women who aren't afraid to learn prudent skills!)
One problem with encounters like that is you may find yourself seething with rage for days or weeks - you have to get that under control. The fear hangover is easier to deal with, since you can reduce it by taking precautions for the future.
So what did you (or your friend) do?