<human>: My 3 year old and I were playing outside yesterday and the new neighbors were out playing too. My son, who is crazy social and outgoing, kept begging to go play with them so I figured, sure why not. 

We introduced ourselves and my son and their two kids played together quite well on their power wheels. And then something happened. One of their kids accidentally rode over the other's leg. I thought it was an accident and not really all that intentional. The father, however, thought otherwise and smacked the kid driving right in the face pretty hard (with my son sitting in the passenger seat of the power wheel). My jaw dropped, as did my sons. I rarely raise my voice (my son never really gives me a reason to raise it, honestly) so for him to see a parent slap was a first. 

I wasn't sure what to do or say. After the slap, my son hugged the little boy as he was crying. I did what I thought was best and told my son he's a good friend. A few minutes went by and all was right in the world again, aside from me feeling quite differently about the new neighbor. 

So I'm walking in town today with my son and an older man walks by. My son did something he never does, he ran behind me. I thought this was weird, but I didn't really think anything of it; I thought maybe he was just being silly. He normally loves to say hello to people and wave. 

And then this evening, my wife is outside with our son and the new neighbors are out. She texts me and says our son is hiding behind her, he's afraid of the neighbor. 

IMO, he's rightfully afraid of the dad. I don't want to dismiss his feelings, but I'm hoping this doesn't turn into a thing where he's suddenly afraid of everyone. 

Any advice on how I should handle this with my wonderful little boy? I know I cannot shelter him forever, but I also don't want him thinking, or assuming, everyone is an asshole and can't be trusted.

Can you condense the text into a few key points?
<bot>: my 3yo son witnesses the new neighbor slap his son in the face. my son hid from a random stranger in town today which is not normal for him. now he's hiding from the neighbor.