If he swallowed some substrate (which would be highly rare and unusual but not unheard of), you should be able to feel it with your finger in his tummy. I suggest you do it asap. If yes, this thread describes how one of our colleaques from Sweden saved his TSN when it swallowed 70 stones with excellent YouTube vids linked: http://www.planetcatfish.com/forum/viewtopic.php?f=7&t=37031&hilit=+gravel
The alternatives could be that he has found some food, unexpectedly for you, or he has internal parasites. The latter may be growing in size/number and filling the intestinal cavity, or causing a digestive clog an/or collection of gas. Another alternative is a tumor.