Don't be discouraged! I'm a firm believer that, especially at this age, brain development is still happening and your son can definitely improve!!!! We had exactly the same problem with our daughter's vision. We kept visiting a opthalmologist with concerns that one eye was turning in, they kept telling us all was fine - for YEARS. Finally when she was 8, we wised up and took her to see a specialist and she set us up for surgery immediately. When we went in, our dd had ZERO depth perception and the doctor warned us that the window for that was probably past - that once that was lost, it would be very unusual to regain it. Now she has excellent depth perception! Her vision will never be great because (we've discovered many years later) she has optic nerve abnormalities due to a metabolic disorder.
Our daughter's hearing was also in question (she had no speech early on and hearing tests were inconclusive).....we sent her to a preK for deaf children so that she (and we) could learn to sign and that was the best thing ever - the physical aspect of signing really helped her speech to develop. So use ANY AND ALL means to focus on your son's hearing and speech development - it CAN WORK!!!! If necessary, search until you find a therapist who is willing to try anything, even unconventional methods, to help your son..........the therapists who helped us the most were the ones who were just willing to TRY - not just think "well this has never helped anyone before, research says it won't work" - you have to think outside the box and work with therapists and doctors who will do the same! Good luck - hang in there - it can happen!
Libby