Is your best the one that also has little clips that tighten and loosen? I have one that is blue that has Velcro straps and little clips. Honestly with Bilbo I just had my sister hold him and put it on him. Once it was on him he calmed down. He just didn’t like being held and restrained. Eventually he’ll get used to the process of putting it on. Are you positive the harness isn’t too tight? If it feels tight around his neck you should bunk up a size. The reason I asked about the straps/clips is because those were the hardest thing for Bilbo and I actually loosened them most of the way then when they were clipped I’d tighten them to where they were close to his neck but not pushing on it. I also, before I actually put him in the harness, would lay the harness/vest loose on his back kind of like you would for a blanket on a horse before you just strap the saddle on. Might try that. Although you may consider getting a bigger vest because if it’s right now, it’s only going to get tighter as he fills out.