Oh, I didn't catch that he's already a year old. I agree, freezer camp.
I use a killing cone. I have heard that cervical dislocation is quicker, but I've never tried it.
Whatever method you choose, it's a bad minute at the end of a good life.
This is normal, for a young cockerel still learning. Not easy to see, for all of that. If you do keep him, it will likely even out in time. The teenage learning period is always hard for young cockerels, and those around them.
If you have no interest in hatching and don't need him to protect...