Khandro, he will feel reassurance if you can give it to him. If he hates going to the vets, can you have the vet come to you? That way apprehension doesn't happen. Rest assured he has no fear of the future or even much concept of 'future'. So he won't fear the process. If he's had injections before, he may remember the tweak but he will have had many worse hurts in his time, and if you think back you'll recall he forgets about vet treatments pretty quickly. So given that Piero isn't in fear, and he has his god (ie you) with him giving him reassurance, he will simply drift off into sleep and 'cease upon the midnight with no pain'. It is by far kinder than allowing him to suffer and it will be very sad, but he wouldn't want to suffer. Big hug and look to the time after his passing.