There is no treatment to reverse the damage.
Welcome to Health care magic.
I am Dr. DADAPEER K, an ophthalmologist and I answer health problems related to eye.
It seems from the history that your father was treated for tuberculosis
Combutol is Ethambutol
and it is known to cause side effect in the form of toxic optic neuropathy
or toxic optic neuritis
I can understand your concern but there is no definitive curative treatment for this condition. Further progression can be stopped by stopping the offending drug. In this case I think he would have stopped the drug.
The main treatment is by use of B complex multivitamins to prevent further progression of the loss of vision. But the visual recovery is unlikely.
But with treatment further progression of loss of vision will not occur.
Along with the use of vitamins he need to avoid toxic agents which can damage the optic nerve
like use of tobacco or alcohol in any form if he is using them.
He needs to undergo evaluation by ophthalmologist by tests like visual fields to assess the severity of involvement of the optic nerve.
Hence to conclude there is no treatment to reverse the optic nerve damage
but further progression can be prevented.
Hope the information is helpful to you.