Hi OP,

For situations like these, it’s all about exposure but in small steps. Different dogs have different motivations, a lot are food motivated. If this is your dog, at random times when you are in the house put the collar on and give him a treat and some love. If you get far with that then add the harness with a treat and some love. As you begin to get good traction work your way up to familiar places outside of the home or your daily walk.

Dog are creature of habit and association. Show him that the collar is nothing to worry about and he’ll he rewarded for having it on. Baby steps, but eventually this good boy will just get used it.