Are you sure it is no where in the tank? I thought my RTC ate one of my sponges from my sponge filter for the longest time but later found it buried under a rock when I was moving stuff around in the tank.
I really am truly sorry about your fish, but I can't help laughing at them. Seriously, why could that possibly seem like a good idea? But, the thing is, you hear about it all the time. Give him a little time to see if he pukes it up (if it goes the other way God help him... its not). GL
You could try squeezing it back up and out if you can actually see the outline in his belly, it might be better than nothing. Sorry about this, what a nightmare!