Stomach hardening (and by that I think you mean distension of the abdomen) can be a serious condition. I would recommend seeing a primary care physician
to ensure there is not a serious or life threatening process.
As for what can cause the abdominal distension, there are many causes. (1) The stomach, intestines, colon can be filled with air. This abdominal bloating is often caused by constipation. If you uncle is not having regular bowel movements, then this may cause backing up of gas and make the stomach look bigger. (2) The abdomen can be filled with fluid. This is a common condition known as ascites. The fluid can build up because of three major causes--liver problems (common in people who have abused alcohol), heart problems and kidney problems. This fluid can cause a firm stomach. (3) Fat can build up. Men generally gain weight in their belly before the extremities. (4) The space can be filled with a mass. Cancer of the stomach, liver, colon or pancreas can cause a mass to make the upper abdomen full.
Hopefully this is a simple problem. Unfortunately, as described above--there are many serious conditions that should be ruled out--especially in an elderly gentleman. I would strongly recommend evaluation by a primary care physician.