I would definately take the fish out of the pond and treat it in a hospital tank or tub.
You would want to run a pump with airline and air stone in the hospital tank.
Add 1 TBS of salt to every 5 gallons of water.
Add melfix according to bottle's directions.
Look closely at the ulcer and see if you see a worm sticking out of it. If you do see one, it will be elongated and and almost look like a piece of fine string. You will have to catch your fish and have someone hold it in a wet net or cloth while you or someone else takes a tweezers and gently but firmly pulls the worm out.
Get some iodine and dab on the hole where the worm was. If your fish does not have a worm but has a flesh wound only, continue to treat with melafix . If he does have an anchor worm then your whole pond will have to be treated for parasites. I use something called Parcid but there are other brands.
Feed a medicated oral antibiotic fish food to the fish in the hospital tub, for a week or so. If you fish wont eat it, try soaking the food pellets in garlic for a few minutes then try feeding again, twice a day.
Keep us updated and good luck to you!