Good for him and good for you! Chihuahuas with cardiac disease can live a long time if properly managed. The little buggers can be difficult to give oral meds and you might ask your vet to teach you how to give an injections if the need should arise.
There are several medications that can help your pup. Lasix is the first in the line for cardiac disease and does a good job. Don't be afraid to ask your vet questions, and it never hurts to get a second opinion.
Glad he is doing better. You will have good and bad days. And remember he doesn't realize he has a serious condition, but he does sense your emotional state. Be brave and he will, too.