Our son is 6 years old. One of the other kids in his class is a bully.
This other boy (we'll call him Walter) likes to sneak up on my son, and physically hurt him. Sometimes Walter will punch him in the belly. Other times Walter will grab my son's hand and bend it back at the wrist.
We have discussed this issue with one of the assistant teachers (the teacher is rarely there when we are), we started with "our son claims that another student has been punching him. We told him that he should tell the teacher, and he says that he has, but that the other student then tells the teacher that my son was the one who was misbehaving." The assistant teacher responded with "Is it Walter?" to which we said "yes." She then said "I'm not surprised. He's sneaky". So it seems very likely that this isn't just a "he said, she said" type of situation. We've also heard similar reports about Walter's behavior from other parents.
We've spoken with the teacher repeatedly, and she tries to keep them separated, but that seems to be the extent of what she's willing to do. They'll be leaving to go to different schools soon, so attempts to escalate the matter within the school are generally met with "it will resolve itself soon".
My wife and I have been working with my son on how to handle it, and we've started him on martial arts classes to give him more physical confidence, so I'm not worried about that aspect of the issue.
However, Walter's father occasionally tries to be social, and has invited my son to Walter's birthday party. There's even been talk of play-dates.
I've listened to this man brag about how well-behaved Walter is, and how smart he is for knowing to keep what he's learned from his wrestling lessons out of school, so it seems his father is completely clueless about Walter's actual behavior.
There's no way my son is going to Walter's birthday (I asked him, and he doesn't want to), or over to their house to play.
I'd really like to let this guy know what his son is really like, and describe some of the bullying behavior. What is the best way to broach this subject with the father?