The absolutely first and most important thing for you to do, is to buy a proper tank for this fish, a 20 gallon would be a good start! It would allow you to add a friend for him Goldfish really enjoy company! But if you're sure that you don't want any other goldfish, a 10 gallon is alright. Just be sure to have good filtration, something that turns 5 times the tank volume would be nice, 10 times would be even better! In short, get a filter that turns 50-100 gallons per hour. You can later adjust the strength of the outflow if necessary. You don't want to blow your fish away after all!

Second of all, get rid of the catfish, you shouldn't ever keep suckermouth catfish with goldies, since they can start grazing on the goldfishes' slime coat. Which of course isn't a good thing. It can cause severe infections and wounds on the fish, which can in the worst case be fatal.

The main problem with your goldfish for now, is that he's probably starting to react at the nitrates, which are higher than he's used to thanks to the new tank mate. So please get him a proper tank asap!!!

Adding the catfish is where you went wrong to begin with.  It put an overload in the tank and I bet you had an ammonia or nitrite spike.  In other words the good bacteria in the tank could not keep up with the wastes of the 2 fish so the eco-system broke down and ammonia built up in the tank very quickly.
Do as the girls suggest and if you want to keep the goldfish then get him a bigger tank.
That smaller tank would be fine for a betta!
As for the catfish, he will need to find a new home because he will get huge and then become territorial.
Now that he is small he and the goldfish probably could live together but as he gets larger you would have to have at least a 30 gallon tank just for him!