Please, is there any way I can save my cat?

He's such a strong boy! He's been through so much and we have to put him down in atleast a week. He has asthma, hiv, and heart disease. I took him into the emergency room and they said it's probably worse. They said he probably has a bacterial infection or something they can't pick up. They said it would be torture to keep him alive any longer because his lungs will get too constricted. I've been giving him medicine every day and he seemed worse before, but today he seems fine. It seems a bit hard for him to breathe but he's active, eating, playing, responding to my touches, drinking, gets excited.. Before he developed all of this the vets always said he's the healthiest 12 year old cat they'd ever seen. I don't understand why this is happening to him. My world feels like it will be nothing without him. I love him so much and I don't want him to go. He's my best friend and my child. My dad suggested getting another cat but i'll never want another animal again. I love him. Is there any procedure or medicine I can give? The doctors said I have the strongest medicine for an animal. He would be fine, but because he has a type of aids, it prevents medicine from working... so medicine barely works for him. God, I need him to get better. Is there a chance he can survive?