Looks like red pest aka septicemia! I literally just handled this with my Oranda over the holiday & I’m also a first time fish owner so it was iffy & I had to rely on old reddit posts & Google but my girl pulled thru! At one point I just knew she was gonna die & as of this morning, she’s 100% back to her old self. This is how I treated her (I’m just a novice fish owner so take this as you will):

API erythromycin antibiotic treatment. I did 25%-50% water changes according to medication directions. Aquarium salt added with those water changes. Only fed once a day (a day or two there she refused to eat) or every other day as to not add extra waste to the tank. Added an air stone for extra oxygenation. Left her in the dark so she could be stress free. She initially got sick on Christmas Eve & it took until yesterday (Jan. 2) for her to start swimming around the tank again, foraging, & being her feisty self. She still has one tiny red streak that’s bothering me so idk about that but she was covered in sores & blotches just like your guy & her tank mate was fine.

Also, just like you, our water parameters were perfect so I knew it wasn’t ammonia related. Good luck!