Signet Cell Colon

My husband was diagnosed Signet Cell colon cancer in Oct 2008.He had a foot of his Colon removed. Had a pet scan everything looked good. His CEA number keeps going up. He has had 3 more colonoscpys. Nothing. Has had catscans everything is good.He didnt have any Chemo or anything. I have heard that it is a very aggressive cancer. Any ideas?