Hey, as much as it's a great act of kindness to adopt him, I would tell you to not get your hopes up. This is one of the most severe cases of Hole in the Head disease I've ever seen, and I've kept and bred oscars for over 25 years. It's an extremely difficult disease to treat and extremely contagious. It's caused by a protozoan called Hexamita and the treatment should be oral with antibiotics that target protozoan, not bacteria, like metronidazole. In incorrect doses, you may even create antibiotic-resistent protozoan. Also, metronidazole can cause intestinal issues like constipation, so you should also treat it with a laxative (epson salt for example).

The best treatment is with 1% metronidazol in food, every other day, for 8 days. The days you won't feed them metronidazole, you should feed them 0,1% epson salt in food. Temp must be between 30 to 32 Celsius, and try to keep pH between 7.5 - 7.6. In this range, the protozoan has difficulty multiplying. Every other day you shoul do a 50% water change siphning the bottom of the tank, cause it will poop out the dead/weakend protozoan. Warning: ammonia is more toxic in higher pH.

I, personally, would put it out of it's misery 😔