My puppy was too small for any of the xs puppy harnesses so I bought a soft ferret harness that fit him perfectly. He hated wearing it as well at first because he wasn't used to it and kept trying to chew on it. I kept it on him for a little while each day and he finally got used to it. I then incorporated the leash and let him run around my living room with the leash attached so he would get used to that as well. Now he doesn't even notice the harness and he's finally big enough to wear a REAL puppy harness hehe
Hope this helps!