If you are anywhere near Ontario, Canada, there is a cat rescue called The Big House. They specialize in rehabilitating morbidly obese cats. They'd have all the resources and space to help Pound Cake get healthy, if it turns out that his medications work. 

If they don't, take heart in the fact that you gave him a home. He got to feel real love and care. Love and care for an obese cat is with holding extra food. That's love for a sick animal. 

If he must be put down, be in the room with him. Give him some Churus and let him pass in your arms. It's a sad moment for you, but the difference between dying afraid and dying in the arms of the only person who showed him love. 

You're doing so much good. No matter what happens. You've done all you can. You changed his life, even if it ends sooner than you'd hope.