My Dad Is 80 Years Old. For Several Months Now,

My dad is 80 years old. For several months now, he has been complaining of gas, constipation, nausea, and loss of appetite. He has also been steadily losing weight, about 50 pounds over the last year or two. I didn't realize the extent of this condition because my mom has been so ill and we've been focusing so much on her. He's been to PCP but they just give him nausea pills, tells him to get more fiber, prescribes Maalox and the like. What could those be symptoms of?