I'm sorry, I know that the general consensus is completely opposite my thoughts but I still think that you are being perhaps a bit (purposefully) obtuse about this whole situation.
Someone (ultimately, does it really matter WHO?) from their family has called your house TWICE and yet you stubbornly refuse to call them. This makes no sense at all.
Also, as much as it kills me to admit that I've given this more thought than it takes to read the thread, unfortunately I have...
It's not like a "simple" tug would be enough to break a zipper or tear the sort of cloth they make anything other than "fashion" jackets out of - certainly anything worn for warmth in the Swiss winter (unlike the Florida winters I'm accustomed to) is made with some sturdiness to it - particularly if it allegedly cost in the ball park of 400chf. Even my cheap canvas coat that came from Tekko (whatever the name of that place is) for 50chf isn't that easy to tear.
So, I think you are taking / making your son's involvement in this whole thing a bit too light. It takes some good force to break a zipper.
I think there is more responsibility to be placed at your son's feet than you care to admit - no matter how many of your fellow EF'ers are patting you on the back about standing your ground regarding a dang telephone call.
Your son pulling at someone's jacket with that much force for retaliation for a playful punch in the arm seems unreasonable to me - so did the boy give your son a good whalloping or is your son prone to unfitting reactions (or maybe, as happens with plenty of growing boys, he doesn't know his own strength)?
I think the only way you may find the answer to that is to contact the school to inquire about the
fight - particularly since you refuse to contact the boy's parents yourselves.