Please take him to a diffent vet.

The one who fixed him may have made a mistake he doesn't want to admit. A second opinion before you put him to sleep would clarify that. Not only that, the first diagnosis may have been incorrect and the medicine might be making him sicker. We know doctors make mistakes all the time! But if the second doctor agrees, then I would agree that you certainly don't want to let him suffer, and putting him to sleep may be the only choice.