I'm very sorry to hear about this difficult situtation. Unfortunately, once the optic nerve is damaged it cannot be repaired to restore vision in the affected eye.
The optic nerve is part of the central nervous system, just like the spinal cord. When people suffer spinal cord injuries, they are left paralyzed permanently because these nerves cannot regenerate and restore their function. Similarly, once injured, the optic nerve cannot restore its function.
I would recommend that you obtain a second opinion so that the optic nerve can be carefully evaluated once again. I assume that a CT scan or MRI has already be obtained, so these test should be taken to this visit so that the doctor will have all the data needed for a complete evaluation. This second opinion may offer some alternatives or at least reassure you that you have been given the correct diagnosis and prognosis.
I hope this helps and good luck as you deal with this very difficult situation...
It is difficult to accept this... I know. I have seen these situations before and I try to reassure patients by telling them that we are truly fortunate to have been given two eyes. Losing one eye still allows us to live a full and complete life with few limitations. During the early stages he will go through many emotions including anger, denial and depression. During this period he needs all of you to be strong supportive and understanding. Eventually there will be acceptance and he will learn that life can be completely fulfilling and rewarding. If you feel that he could benefit from visiting a child psychologist to help him cope with this, don't hesitate to facilitate this. It can be extremely helpful.
Again I wish him and all of you the best... good luck