|When we arrived at the gym he ran in to play. C is a boy we have seen quite a bit but do not know well. I don't really click with his mom but we are in a small co-op together. So C was playing with another boy with a ball. The ball fell and I guess DS picked it up and ran with it (obvious to me it was his way to initiate play). C was upset, chased DS and called him "Weirdo" over and over again. DS lost his temper and hit C over the head with the ball (one of those soft rubber baby balls, NOT a basketball). C then hit DS, DS hit him back.
I'll preface by saying my son struggles similarly.
Your son interrupted the game of another two boys, and likely to the other boys it seemed that he was stealing the ball rather than trying to join in. Thus the enraged "weirdo" and chasing. Your son then escalated things by hitting the other boy; back and forth hitting ensued.
Here's some hard-earned advice, not meant as judgement, but just what I've learned and what's worked for us.
I know your ability to quickly respond was limited due to nursing at the time, but knowing that my son had trouble joining in play, I wouldn't nurse until my son settled in (if possible).
Assuming I was free to move, I would have gone over and gently separated them. I would have said clearly to my son "you may not use your body to hurt another person" (established rule, and stated this way addresses both of their behaviours). If the boys were clearly not going to be able to reconcile then, I would have had my child come to the side with me for a discussion about choices made, what he could have done differently, what the other kids probably thought he was trying to do (steal the ball rather than join play), and what he could do to make it right.
If the boys looked like they could play together, I would say to the other boy "it seems there was a misunderstanding. Billy grabbed the ball because he wanted to join you and it looked to you like he was stealing it. Is there a game the three of you can play together, without hitting and name calling? How about ______."
Later, DS and I would debrief about what happened. This is an area my son struggles with. I view my job as coach for him, and to support him in scenarios that are predictably challenging or vulnerable for him.
I have nothing to say about the other mom; I hope she was just having a particularly bad day. Though it seems she has a different perspective on the kids' interactions generally.