Well first I want to say so sorry that a "Boxer" started this cycle, obviously that owner is not on "Boxerforum" or I'd have run across them and it would not have been "good!"
I'm not really understanding how this can be a "Breeder" problem myself?? Not say it's not just saying I don't see it. With my dogs my first puppy Stewie APBT/Boxer got "targeted" once. My American Band Dawg Gunther got targeted once, I was not on seen but no contact in both cases. And my Boxer Struddell never had a single "event."
So that brings me to my OS WL GSD ... yeah ...he seems to be a Stray Dog Magnet??? So far 7or is it 9 "attempts" on him thus far?? I lose count thus far all have persuaded to move on without any "actual contact." It's kinda odd but what are you gonna do??
Hmmm .... that's actually a rhetorical question, here are a few things members have suggested to deal with "stray dog encounters.":
It sounds like you kinda sorta did not know this could happen?? I don't put a lot of faith in other dog owners myself, that kinda helps.
Moving on ... the PO thing yeah that mostly would have worked out fine, still can ...but now your dog has "issues" and trainers with that particular bend ... are not going to be to much help to you?? Right now you need to restore your dogs faith in you and cookies and treats ...aren't going to get it.
Trainers that can help you look more like this:
Not in conflict with "Steve's" advice just gonna expand on it. And to be "clear" your dog does not have to associate with other dogs, he only has to learn to be "civil" around them. "Sit on the Dog" and "The Place Command" would be how you would get there, you do "Sit on the Dog" and "Train" the "Place Command." Eventually your dog will be able to do those things in the "Presence" of other dogs without "losing his cookies" as it were. At some point after lots of work on them ... out could start to do them "Outside" the confines of a Dog Park, if you want to "Proof" him. Or don't that's your call.
In anycase see here:
Fearful, Anxious or Flat Crazy "The Place CommanD - Boxer Forum : Boxer Breed Dog Forums
That would be the "foundation work" and for more on "teaching your dog to ignore other dogs ..." see here:
Teach your Dog to ignore other dogs. - Boxer Forum : Boxer Breed Dog Forums
I don't think you need to contact the "Breeder" for this?? You only need a plan and now "I think" you have one?? As always ask questions and welcome aboard ... sorry it's been a bumpy ride thus far.