You obviously love your kitty very much. I think that if you have to amputate he will do fine. My family did rescue and rehabilitation with animals since i was very little. We have had many cats that came through missing noses, legs, tails, eyes, ears, and toes. You will be surprise how well they get around. I know he will want to go outside but until he gets a good handle on his movements you might want to wait before letting him back outside. make sure he gets lots of love and treats while he recovers.
I do not think he should be put down if he is only facing an amputation. Cats are resilient, feisty, and adaptable. If he is skittish, don't crowd him or force him to accept a touch while he heals. He'll be scared and in pain for a while so move slow, speak gentle and let him cue you as to what he wants.
There have been a couple of feral cats we have had rescued that suddenly became lap babies after major surgery. They realized that they got pampered when hurting, and that was that.
My old black cat was a feral when we got her. She died two years ago at the age of 19 (we believe that was her age). When we found her she was pregnant, wild as the day is long, and near death. Someone has shot her with a shot gun, and her leg was shattered.
She didn't have to have it amputated but that leg bothered her until the days he died. We kept her indoor as she healed, and one day she got in one of our laps.
Then another day she decided she liked us. And when she decided she liked humans she REALLY liked humans. You had to pry her out of your lap. She was still the cranky war horse we knew who would growl if you pet her incorrectly, and nail you if you touched her arthritic leg, but she loved to be in laps. She had a hard time using that leg but she was still a great hunter as well as stayed boss of all the other cats.
That cat was something. I miss that cranky old girl.