It's a bit difficult to tell from that picture (well done for trying though!) but it sounds more like an injury than an illness.
I'd suggest you check his tank over for anything that might be sharp, and get a guard for your heater if you haven't got one already.
For the moment, I wouldn't recommend a medication, but daily water changes as a precaution, and keep a close eye out for any secondary infections, like fungus.
If it was my fish, and I have done this before when I've had fish with small injuries, I'd do a 25 or 30% water change with warmed but not dechlorinated water; use the chlorine in there as a mild anti bacterial, that is what it's in the tap water for in the first place. That amount of chlorine won't be enough to damage your fish or your filters, but it can help stop fungus from attacking the site.
Your plants look amazing, btw; wish mine looked as good as that!