Your son’s story doesn’t pass the smell test very well IMHO. So He’s buddy buddy with this guy, they’re horsing around like buddies do which includes:
1) Saying You little bitch (seriously we used to call each other this as a joke, no harm was meant and it was understood)
2) Saying yo-mama jokes to each other (We told off color jokes to each other when we were kids, didn’t go telling the parents about it)
All this is good and fine, but for some reason the other kid tells his dad about him and his “buddies” good natured ribbing.
You see how this doesn’t pass the smell test very well? Middle school guys don’t go home and tell their dad about how their buddies horsed around that day, that makes no sense to me.