The technical answer to what they are looking for is that a CT scan sees differ4ences in tissue density and cancer tends to be denser than normal tissues. In your husbands case they are probably looking for a reason for his weight gain. There may be a number of things going on. The first thing that comes to my mind some sort of intestinal blockage that would explain the weight gain and the hard and distended stomach. But it could be a lot of things. The symptoms you describe are to my knowledge not typical for metastatic prostrate or kidney cancer so I would not tend to immediately think that this is necessarily a sign that the cancer has returned.
As for the feeling that the Drs are not looking at the full picture, does he have a lead Dr that is supposed to oversee and coordinate all of his treatment? This is usually done by a medical oncologist. If he has one then you should raise your concerns to that Dr and ask who is responsible for looking at the big picture. They should take the time to sit down and explain everything being done and what the know and suspect. If the Dr seems rushed try making the last appointment for the day so that they can't use the excuse that someone else is waiting. You have a right to an explanation that you can understand and to know the details of what they know and suspect. Good Luck and let us know how it goes.