How do I prove a cabbie tapped into my bumper?

So I was driving and I went to pull up to an intersection. Just as I was coming to a stop,my car jerked forward lightly but definitely noticeable and I slightly let off the brake. The cab driver then started tapping his brakes like he didn't think they were working properly. I thought he would follow me, but he turned the other way. I got the first 3 digits of his plate and I know the company and I called them but they deny it because there is no damage on the van. I took pictures of the damage and it's minimal but I'm still furious he didn't try to catch up with me. I have pics of the rear of my car from last May, and you can see the differences. Any input helps!