He does look skinny but may be the 'runt' of the shoal that gets least access to food.
If his spine were bent, I'd be concerned that it might be Fish TB, but that doesn't seem to be the case, fortunately. As there are no other symptoms, no medication/treatment would be of any use, and medicating as a precaution much more likely to cause harm than good. [Edited to add: internal parasites is a possibility, though, so worth keeping an eye out for any other symptoms that might develop over time.]
I like to feed Hikari First Bites which disperses like a powder into the water column in such a way that all fish get access to food. In particular, I've used it with very juvenile fish, elderly/ailing fish that struggle to eat, and in cases such as this to ensure that any fish that might otherwise miss out on sufficient food do get more.