[Maximo]

Did your vet really use the word "aggressive"? I would change vets.
1. Koda is a puppy
2. How long did the vet have to observe Koda -- 20 minutes in a small exam room?
3. Vet visits are stressful for many dogs

Sounds like the vet was trying to blame Koda for his/her inability to make Koda feel more comfortable.

Regarding Koda's resistance to the collar, this is not uncommon. Many dogs will shut down when something unusual is placed on them, like collars, clothes, hair bands and bows. They require acclimation time. Use treats, praise, and encouragement to help Koda like the collar. Distraction is also helpful.

Is Koda old enough for walks? I used walks to help my boys like harnesses, hair bands, and clothes. They learned quickly that wearing these things meant going outside for fun!

For training, keep the sessions brief and always fun. If Koda views it as a chore, he won't be interested.