sore or ulcer.
about the only medication that you can use on seahorses is triple sulpha (tri sulfa). Probably best to put him in a quarantine tank for treatment.
If there's no improvement after a few days with the sulpha based medication, you might need anti-biotics (assuming you can get them).
Another option is to put a couple of drops of Betadine on the wound, but that means lifting the seahorse out of water, drying the area, applying the Betadine, waiting 30 seconds and then putting him back in the tank. This would have to be done 2-3 times a day for about a week and would be stressful to the seahorse, especially if he gets air in his pouch while out of water.