If it was infact just scrapes, it's more than likely a secondary bacterial/fungal infection which has caused the ulcer to manifest - most likely bacterial. Skip the FW bath (if you were even still considering that) because it won't do anything for an infection. You'd be best catching him an directly applying an antibiotic ointment on the wound, but if that's as impossible as you say, you should look into a broad spectrum antibacterial/fungal med, and always always start with the small guns! Too often, in their fear, people rush for the heavy meds when they aren't needed. Heavy meds can work miracles - yes, but they are also more dangerous, come with serious risks/stress/injury to the patient (your fish) and any other co-habitants, and can - when exposed too often or unnecessarily, cause undesireable immunities. So what are the small guns - Pimafix and Melafix. Run a full 7 day treatment of a cocktail of both, up your aeration and lower the temp slightly on the tank, and be prepared for the water changes. If after a full course the ulcer is still apparent, and hasn't seemed to improve, you might want to opt for a stronger med. If it HAS improved, just hasn't completely healed, do your water change, replace your carbon, and add a conditioner to the water to help aid in slime-coat replenishment and give it another 2-3 days for healing (so long as it's eating, swimming and behaving normally and doesn't appear to be in distress.)
Like I said before though, if those meds don't work, you'll need to keep an eye on it and be able to recognize the telltale signs that you need to up your game (to avoid becomming systemic). If that were to happen - the need for heavier meds, I'd sugges Metronidazole and/or Nitrofurazone. Both work really well in marine environments, and are awesome at bacterial infections and some virals. When I say "AND" yes - they can be mixed, but DON'T do that just because you can! Like I said before - less is more, and for this, you don't need a cocktail of those two especially if you've never dosed with those and because right now, the fish is still in good health - all things considered.
