There are a couple of possibilities for an enlarged liver, but given the evidence of heart enlargement, I would have to assume that the liver was big secondary to right-sided heart failure.
Usually, this means that there is a problem with either the tricuspid valve (the valve between the right atrium and right ventricle) or the pulmonic valve (the valve from the right ventricle to the lungs). It can also occur due to things like heartworm infection (not sure if this is seen in Fiji) or severe lung disease. Any of these conditions can result in right sided heart failure, meaning that the heart has been so weakened that it is no longer able to efficiently pump blood from the right side of the heart to the lungs. As a result, there is fluid back-up in the right side of the heart, and consequently the vena cavae and the liver. With fluid back-up in the liver, we see liver enlargement and can also see ascites, which is fluid within the abdomen.
Less likely, is that the use of prednisone caused the liver enlargement. Generally, steroid hepatopathy (liver disease secondary to steroiid usage) occurs as a result of chronic, long term prednisone treatment. One month of therapy is usually not enough to cause much of a problem, unless very high dosages were used.
However, steroid therapy can further weaken an already ailing heart, so use of the prednisone may have exacerbated the heart disease.
You did not respond to the question regarding whether or not your boy was diagnosed with congestive heart failure (fluid within the lungs). If this is the case, then the heart problem is more likely associated with a mitral valve problem (the valve between the left atrium and ventricle). When this valve becomes faulty, the left atrium begins to enlarge and eventually gets to a point in which the heart is no longer pumping effectively. As a result, fluid builds-up in the left atria and increasing pressures here results in the development of pulmonary edema (fluid within the lungs) and congestive heart failure.
The enalapril is an ACE-inhibitor designed to lower blood pressure to decrease the work load of the heart. The furosemide is a diuretic, which means it helps to relieve the body of excess fluids. This medication can be given for cases of pulmonary edema, and is often also given in cases of right-sided heart failure.
A third medication, that has been shown to improve longevity and quality of life in dogs with heart disease, particularly those in congestive heart failure, is pimobendan. You can ask your vet if it is available in Fiji.
It is not surprising that your boy seemed well one day and developed problems the next. This is common with heart disease in animals. Typically, they are able to maintain themselves despite a weakening heart, but eventually get to a point where they are no longer able to keep things in balance and quickly go downhill.