It's quite difficult to tell from the pic, but fish can lose scales due to injury. If he is eating and behaving as normal, then he's probably not suffering unduly. If you see any fuzzy white growths on it, then he may be getting fungal infection, secondary to the first wound, if it's a wound. If it's not a wound, I can't imagine what it might be.
In any event, there is not much you can do other than keep the water as clean as clean can be. Extra water changes to help prevent the chance of fungal problems if it's a wound, and if it is missing scales, it should heal with time. I'd have to go look it up to find out if fish can grow new scales but they can grow new fins and barbels, for sure. I'll go see if there's any info on scales.
Edit. a quick search tells me that scales can regrow, and if they were just knocked off by some sort of bang or scrape, they should grow back. If there is a lesion, that is, a deeper wound, there is some chance that only scar tissue will grow, and not new scales. Only time will tell.
Keep the water sparkling and keep an eye on it for any signs of infection from fungus or anything else, and hope for the best. There is often very little we can do for injured fish, beyond keeping their water as pristine as possible. If you can get him some live or frozen foods, it should assist the healing process, being higher quality proteins and fats than most flake foods offer.