Others did it and I did it - reach in gently and pull it out, I'd say. Just don't cut him too bad or rip the flesh but even then, he will likely survive. If you know how - sedate him say with clove oil.
They usually regurgitate what they cannot digest or take a long time to pass a foreign object or, when they cannot do either, they eventually die. Sometimes people successfully extract things from their stomachs (when they cannot throw it up and neither can they pass it in their intestine). Once in the intestine and they cannot pass, it is either a surgery ($$$) or death.
A couple of examples of successful extractions with nice vids:
http://www.youtube.com/watch?v=JG5Dz5Iwol4
http://www.planetcatfish.com/forum/viewtopic.php?f=7&t=37031&p=253193#p253193