i think gizmo summed this up so nicely with the "its the last thing you can do for them" comment. It is a tough decision to have to make for our pets, but what you have to remember is that they do not understand quantity of life, just quality. To be blunt, i think nappies and carts for bunny rabbits are plain cruel and selfish, this is an animal who cannot be reassured and comforted in the same way as a human, or even a dog. THese people are keeping the animals alive for their own feelings (its tough not to, i know, i have had to put my dogs to sleep and i was devestated but i KNEW it was the right thing to do)
I have worked as a veterinary nurse and i can promise you that the procedure itself is stressfree and painless for the animal. It is about letting our pets go with the minimum of stress and the maximum of dignity, also for your own peace of mind. That way, you have time to adjust to what will be happening and can say a calm goodbye to nibbles. Of course you will then feel like the biggest shit on planet, i did and i basically hated myself, but time will out and i KNOW i did the right thing for my dogs. I would definately do this BEFORE any discomfort to the bunny, as you are only buying yourself a few days or weeks by the sound of things.
Your bunny is lucky to have you as an owner, i have had to deal with some awful states wehre the poor rabbit is just neglected in the hutch until the poor thing is in obvious pain because the owners simply havent noticed, or bothered as they are out of sight out of mind.
You will make the right decision for your bunny. Go with your gut feeling, this is what i always say, your heart will not want to let you and your head will try and rationalise it, as i can see it is, "oh but he is not suffering yet etc" You have to follow the feeling at the pit of your tummy.
It is hard for your son and you i know. I think