Use a magnifying glass to look at his flesh closely to see if you detect any parasites.
Start feeding the fish 3x a day with meaty foods soaked in freshly minced garlic. Mince up the garlic in the same dish that you will place the meaty foods [no flakes] then place garlic food in the fridge for 30 mins to give the garlic time to soak in real good. You must preserve the garlic juice, so make sure you mince the garlic, then place the food in the same dish that you did the mincing. If you can use seafood such as the kind you find at the seafood counter at the grocery store, then better. Food process the seafood with a chunk of fresh garlic.
I can't promise that this will prevent an all-out outbreak but its worth a try.
Think about setting up a Quarantine. If ich breaks out, then you will need to treat the fish with hyposalinity. You can remove your rock and place it in large rubbermaid container with a couple of good powerheads going.
What inverts do you have? Inverts will also have to be removed.