It is infected and hard to see past the hair and drainage. It may be a scratch or puncture from some form of trauma either rubbing against something, a bite, or a hot spot. It does need to be seen by your vet for wound care and antibiotics. Until you can get him in, you can clean the area with soap and water to help flush the discharge out and clean it out. Then apply an antibiotic ointment such as Neosporin to keep it clean until you can get it examined