I'm wondering if the mailbox could be moved out to the road so the poor guy doesn't get cornered on the porch again. I know UPS and Fedex aren't supposed to use the mailbox, but they toss my stuff over the fence.
I don't know what your mail trucks look like where you are, but here, they are closed-top jeeps with no doors. The newer ones are essentially ice cream trucks with no doors and music.
If I were a mailman and had dogs barking at me, chasing me, cornering me on the patios, and I had no doors on my mail truck, I'm sorry, I'd be seriously acting just like your mailman and saying bad thinks to your neighbor. I'm just coming from the angle of a person who is out there doing that every day with minimal protection.
It may or may not have been intentional. It doesn't matter, though. He comes by almost every day, so the odds of him hitting loose dogs is much higher than other delivery services hitting loose dogs. If FedEx had hit 3 of your dogs, my eyebrows would raise, but not much. Either way, look at it from his side. IF he accidentally hit the dogs, imagine how that would feel to someone. IF he purposefully hit the dogs, imagine what would push him to do that. You outlined quite a bit of it. I don't think he got this job with the hopes of running over dogs. I'm just saying, have a little compassion for the mailman who has had it up to HERE with being chased and attacked by everyone's dogs.
And good luck this evening with your son.