You could try vitamin-soaking his food and offering a more varied diet. HLLE is often diet related. If it's a slight bacterial infection the vitamin will also help boost its immune system. You can try giving him a fw dip to rule out flukes.
If the marks get worse/red or start to spread he might need antibiotics. Preferably as a bath in a QT, but if that's not possible you can bind it to food and try to get it into his bloodstream that way.
If it's ich, or more likely velvet, you'll need to get everyone into QT for treatment and a 76 day fallow period. And you'd want to move fast. You would expect to see other symptoms like scratching, breathing heavy, swimming into flow, yawning, loss of appetite, etc with these.
I'd research a little more or try to get a clearer picture so we can help narrow it down for you. This is a good place to look to see if something matches:
Fish Disease Index - Pictorial Guide
https://www.reef2reef.com/index.php?threads/Fish-Disease-Index---Pictorial-Guide.285708/