The best thing you can do is understand that kids can be kids. There are good kids and bad kids. There are kids who, like you said, pick kids who they can control. And then there are kids who refuse to play that game, like your son.
If I were you, I'd continue to do what you are doing, and additionally, get your son some martial arts lessons.
Do you know the parents of the children who were trying to ambush him? (Other than the policeman's son.) I wouldn't go into the amount of detail with them that you have with us (they might think you are a bit obsessive about not liking this kid), but you might let them know that they need to talk to their kids about aiding a kid who you think was planning on beating your son up. If a parent came to me when my kids were young, explaining that to me, I would take it seriously and talk to my kids about it. Just knowing that a parent is watching can make a difference the next time the bully tries to enlist help.
Of course, it's upsetting to us for our children to be bullied. And you have a lot of pent up anger and resentment. But take care not to subscribe adult behaviors to a kid. He IS a child. It's a shame that his parents are unwilling or unable to control this. THEY are the ones you should feel more aggrieved at, to be honest.
Dawn