I'm not a trainer, but your trainer might be saying this to make sure EVERY human encounter is positive. Your dog might not view petting as positive, it might be scary for him, thus the reactions he's been displaying. To help build his confidence, it might be important for him to start seeing human interaction as a good thing.
While you work on his issues, I'd avoid situations that might end badly for your dog, including the dog park. Set him up for success.
This sounds like a delicate issue, and it'll take a lot of time. I'm no expert by any means, but that's what it looks like to me!
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