What Causes Distended Abdomen Along With Severe Constipation?

My dad who is 75 years old today noticed that part of his stomach felt like a lump and had moved down and to the right. He is a veteran and will only go to the VA for treatment. He has many problems primarily a severe back problem which has him on Morphine which caused him a lot of constipation. He is not overweight but has always had a pronounced belly. Any thoughts?