Ugh! That's awful.
Water changes, water changes, water changes. Load fresh Activated Charcoal into his filter, this will help soak up the soap. If you don't have a filter for him, Tetra makes a really small one that will probably fit in a bowl. Use an additive that helps fish build a slime coat. I'm not sure how to clean the stuff out of his system, though.
Honestly, his chances don't sound good to me. Soap clings, and it makes it difficult for the gills to absorb oxygen.
To keep this from happening again, I'll go over some "best practices" (ugh, I've been working for Taco Bell too long) with regards to water changes.
-Don't move the tank when you do the water change.
-Don't remove the fish from the tank when you do the water change. Since you don't want to do more than 50% of a change at a time, he'll do fine if he stays in the tank, but taking him out stresses him.
-Use a siphon to pull from 10-50% of the water from the tank, then add treated water back into the tank.
-Try to disturb the water as little as possible when doing the change, this will minimize the betta's attempts to leave the tank.
Good luck to you and your fish. Ask if you have any other questions. Like Caine, we're here to help.
