He might have fin rot, SBD, and velvet. Do a 100% WC and make sure the temp is 80 degrees.
Oak leaves are great for fin rot, but make sure the leaves are brown and naturally fallen and don't have pesticides on them. If he's bloated, then fast him for a couple of days.
As for velvet, I would use 12 drops per gallon of bettazing and add aquarium salt. Don't use bettazing unless you are 100% sure he has velvet. If he looks like he's covered in rust mist, then he has velvet.
To be perfectly honest, I would :( I know its really hard but if you don't think he'll make it then its probably best to put him out of his misery. But if you think he has a chance to pull through then I would keep fighting.
Medication is really hard on fish and its possible he won't pull through anyway. It really all depends on the fish.
Sounds good. In the meantime try to diagnose what he's got going on.. ich, velvet, parasites, columnaris, dropsy, etc... I'd recommend doing a treatment of Jungle Fungus Clear to start off as well as trying to buy some antibiotic foods.
My oh my... if only I had enough money... atm I only have about 7 dollars cause I lent out some money to people... but by the time people pay me back I should have almost 20. Should I buy a couple antibiotics or a new heater (since the current one is rubbish and is only heating up about 74 degrees?)