If your son has a hearing loss of 40-50 decicels, he needs to be fitted with some type of amplification and he needs some one on one therapy to make up for the deficit in access to language for the past 3 years. His behavior problems are a direct result of his fustration and inability to clearly make out sound and speech. This child does not have "selective" hearing, you've been told he has a 50% loss. Get this kid to an audiologist and have him fitted with some amplification and then contact your school district about getting someone to work with him to get him "caught" up.
My specialty is childhood/adolescent emotional, behavioral and developmental problems, so I am not qualified to address matters about the medical implications of hearing loss. However, from the behavioral perspective, it will be important to learn from the ENT doctor or your son's audiologist how much your son's hearing loss should be expected to impede normal day-to-day parent/child communication. You indicate that your son may be hearing better than he indicates at times, and this may be so. But it's best to learn from the specialists and then you'll know better how to proceed from a parenting point of view.