Agree that he does look either stressed or unwell, not just the color but clamped fins, ragged pectoral fins, and somewhat thin. But these describe a generalized condition that could have a variety of causes. Was he bullied or fighting before you got him? Stressed in transport, either by handling or ammonia? Such things could account for what you see, it's not automatically parasites. Could be he primarily needs time and conditioning. There's an argument for quarantine, an option for observation, conditioning, protect your main tank just in case, etc.
But, unless he's still fairly small, a 10 gal tank isn't the best option imo if he's stressed, underfed, in need of conditioning, but not actually sick. I'd be more inclined that way if I could put him in more comfortable quarters for observation, conditioning, and possible treatment. For me, how long would depend on observation more than the calendar, see him fill out some, fins open up, get some color, etc.
If it was me, and everyone has a different approach on these things, I might do API General Cure, usually easy to find-- don't let someone convince you the product name means it's too "general" to do much. The ingredients are praziquantel and metronidazole, effective ingredients, typically mild on fish, found separately in some products with more impressive names. Metro is a go to med for a lot of people treating internal issues and prazi the same for certain flukes or parasites. --my opinion considering you don't have a specific diagnosis, but everyone has their own go-to meds, etc.
How big is he? It's true that growth can vary, but you can ballpark a minimum age by size. If he's in the range of 5,6 inches, he's probably near a year, possibly older if he was raised in sub-optimal conditions.