Here is some info on digoxin...
Digoxin for Veterinary Use
It's wonderful that you were there for him and your quick actions certainly saved his life.
My Bijou was diagnosed about 2 years ago with heart disease. In her case, I disagreed with my vet and took her to a clinic out of state for a complete work up. It turned out she had a respiratory infection and no heart disease.
I'm only saying this as my vet wanted Bijou to take the drugs (sorry, I don't remember the name) for life. She was only 2 years at the time and I didn't want her to take any unnecessary drugs. While it may be necessary in your case, you have to be mindful of the side effects of any drug taken over a period of time. From what I found, most all these drugs do weaken the heart muscle over time so great care must be taken with them.
I hope someone with more info comes on and I wish the very best for your little guy.