Badger, you are a great mom, and he will be okay! You got him to the vet and they were able to treat him. You couldn't know he was worse than you thought, because as you said, he was eating. I'd have thought he was okay too, since I've always heard as long as they are eating they are okay.
I'm glad he tested negative, and I'm sure he will test negative in a few months too.
Now all you need to do is just get him well and spoil him rotten.