Hi Phil! I'm so sorry that Cubby is so ill! You can try giving him Nutri-cal, available over the counter perhaps at your local pet store, but it is honestly unlikely to help him walk again. He is extremely sick from the fluid around his heart, and he needs veterinary treatment now. If you can't afford it, in all honestly, the best option is to let him go peacefully via euthanasia. I have no doubt that you don't want to hear this, but he is suffering and it isn't fair to allow him to continue in this manner. His quality of life is quite poor. I will give you a number of resources that may help you afford a vet so he can have treatment and hopefully have a better quality of life. You can see if your vet accepts payment plans, if there is a low cost clinic in your area, or you can apply for Care Credit online. There is also an organization called Red Rover that give grants for treatment of sick animals, and you can see if you qualify for one of their grants. Best Friends also has a list of resources that offer financial assistance on their website.
http://www.humanesociety.org/animals/resources/tips/trouble_affording_pet.html?credit=web_id91754962https://www.redrover.orghttps://www.carecredit.comhttp://bestfriends.org/resources/financial-aid-petshttp://lessonsfromaparalyzeddog.com/when-pet-owners-cant-afford-veterinary-bills/ God's Creatures Ministry helps pay for veterinarian bills for those who need help.
The Onyx & Breezy Foundation has many programs including helping people with medical bills. They are a good resource for information.
Brown Dog Foundation provides funding to families with a sick pet that would likely respond to treatment, but due to circumstances, there is not enough money immediately available to pay.
I hope one of these resources helps you!