Cysts usually aren't fatal, even if they open up you should then be able to treat with Aquarium
salt but for now to reduce the swelling use Epsom
. Cysts will come and go so this one may go away or burst and then another can come along, same thing happens in humans although it's much more risky to try to remove it from fish.
It can be fatal but usually they'll be fine, just make him comfortable is all
You can still feed him normally as I did mention so today can be fasting and then just feed him as I suggested tomorrow
He'll still need to keep up his strength. Also make sure to take out any uneaten food so it doesn't spoil your water ^_^
So just make sure you dissolve the salt, I use a little cup, measure out the salt and with some tank water in there, stir it around and then you can dribble it into the tank a little at a time. You can do one teaspoon per 15 minutes if you want to measure it that way. And then when you change his water, put the salt in before you put him back (if you're doing 100% change) and then if he's in a cup, float him in his tank and add a Tablespoon of new tank water to his tank every 10 minutes to acclimate him to it
I hope he does well and update or ask questions as needed!