Treatment will greatly depend on the cause, however that looks like a hot spot. It should be seen by your vet, so they can clean it and provide you with some antibiotics and pain meds as needed. Until you can get to your vet, placing an Elizabethan (cone) collar on Hector to stop him from licking and worsening it, and keeping the area clean and dry can help, however if it continues to worsen it needs to be seen right away.