It certainly needs antibiotics. It is trated first empirically. Then, if it nodes not heal in three days, or start healing, then needs a culture with sensitivity. How do you know it is only external? How do you know it is internal? Miiddle ear or extarnal ear? A doctor needs to check it. If it is internal, it comes from an upper respiratory infection, if it is external most likely cause it is from swiming. Most common bacteria from jacuzzi and swimming pool is going to be pseudomona. That can cause ostemyelitis of the maostoides bone if not treated properly. At that point, it needs surgical debridement and IV antibiotics with several weeks of hospitalization.
My order is for you to take your son to the doctor so he can put his otoscope and see if the timpanic membrane is inflamed or it is only external. Did he have a cold recently? Is it externally itching? Does it hurt when you move or pull the ear? Does he have any diminishing in hearing?
He needs ear drops before going swimming if that repeats after treatment, or cover ears.
It can become a serious infection. Take him to doctor. Today.