DON'T put it down! There is a chance. I've seen it before, and sometimes it can be fixed.
Start doing daily, 100% water changes. Take out any leaves, and of course any detritus on the bottom. Do not feed. Do NOT feed. Nothing. Zip. Zero. That can be hard to follow through with if you are an empathetic person. Remind yourself that he is already dead if you don't follow through.
When you are absolutely, 100% sure that the bubble is gone, give him another couple of days. Then begin feeding as a reduced rate for a while. Watch him closely for a while. If he balloons again, you need to do the treatment longer.
This treatment was not designed by me. It's tried and true, by many other members. It's not really a "secret", but it's a nugget of knowledge that could stand to be passed around some more.