This is not necessary. FIV is only transmittable through a serious bite wound. I have FIV+ cats with kittens and elderly cats, it's fine. I'm sorry that this was put to you in a way that made it sound so hopeless. Is it too late? There are rescues that take FIV+ cats. Where are you? I truly don't want to make you feel worse but I feel terrible that this cat is being euthanized when he could lead a long and healthy life.