Call your local ASPCA and explain you have a veterinary emergency. You will likely be required to sign your horse over to the state, and relinquish ownership of the animal. However then he will receive the vet care he needs without financial aspects being a stumbling block.
As tealamutt said, your horse shows signs of a neurological problem. This is not like, worms or a little cold. There IS no at home treatment. We can't advise you to give him six carrots cut into quarters and he'll magically be all better. There is no other advice we can offer you from this kind of forum. This is way beyond what you can to do make him more comfortable. If the horse is having back end weakness and trouble staying on his feet, he frankly doesn't sound like he has long to live unless you put care of your horse in front of your own selfish needs - even if that means losing him, at least it will still be saving his life.
Now, depending on where you live, on RARE occasions the ASPCA will provide medical services WITHOUT making you sign over the horse, and work with you to pay it back, But in these times, I wouldn't count on it.
If you love your horse, do the right thing for him. Don't sit back and wait for him to die.