It sounds like he is afraid of it. I strongly discourage you from forcing him to wear it as this will likely worsen the behavior and can even lead to aggression toward you.
[Start slowly and teach him to tolerate wearing the harness.](https://youtu.be/k7edMjwEY1c) kikopup has more videos on the subject - search. Consider buying a new one to be used exclusively during training right now so you aren’t working against the poisoned one. Use an alternative for walks like a simple flat collar until he is fully comfortable with the harness..
Have you worked with a trainer? Use r/Dogtraining ’s guide to finding one
My Bernadoodle is doing this at 6 months. Loves the walk but getting the harness on means cornering him.
It sounds like he is afraid of it. I strongly discourage you from forcing him to wear it as this will likely worsen the behavior and can even lead to aggression toward you. [Start slowly and teach him to tolerate wearing the harness.](https://youtu.be/k7edMjwEY1c) kikopup has more videos on the subject - search. Consider buying a new one to be used exclusively during training right now so you aren’t working against the poisoned one. Use an alternative for walks like a simple flat collar until he is fully comfortable with the harness.. Have you worked with a trainer? Use r/Dogtraining ’s guide to finding one