Call another doctor for a second opinion. Your doctor's responsibility is to the health of the child. If he won't refer you to an ENT, simply explain that because your son's health comes frist to you, you'd like to seek another opinion. Ask his office staff to forward all medical records to the second doctor's office. If you feel that's too aggressive, ask for copies of everything in his medical file, then take them to the second doctor personally.
Speaking from experience with my daughter and the ear infection nightmare. (We had the same issue with our doctor not refering us to an ENT.), there is no reason why your doctor should have issues with you seeking a second opinion. If he does, is he really the kind of health practitioner you want caring for your child?
It'll be okay. You can do it.