I'm going to buck the trend and say there doesn't seem to be any reason to consider having him put to sleep, other than convenience (and I really don't mean that to sound like a criticism; you're obviously doing everything you can under circumstances not of your making). I believe that when we take on an animal, we take it on for life, in sickness and in health etc. They bring us immense pleasure, and if we enjoy the good times I think we have a duty to stick with them through age and infirmity. Very different if they are suffering, in pain, or dying, then I would consider euthanasia. It doesn't sound as if your cat has reached that stage yet, and although you can't give him the life you would wish you are giving him a better life than many pets enjoy. I would say stick with him, see how it goes, take the vet's advice. You will know when the time has come.