Savvy - what a sad story. But as the vet has said - it is not as bad as it could have been.
Nature has a way of compensating. My Rottweiler dog had a cruciate ligament breakage which meant he had to learn to walk on three legs for a while. A 44 kilo dog learned to balance himself and get about with only 1 back leg. Amazing.
I have lived with the sight of only one eye for almost a lifetime. I can't see properly out of my left eye but I can sense movement and I can judge distance. There are a lot of people like me living with sight only in one eye. Your horse will make similar adjustments I am sure.
What is necessary for you though is to consider how he might be affected
And how you are going to help him adjust to his new state.
He might initially have trouble judging distance - you'll have to cue him better as to where to put his feet.
He might be worried about objects moving on the side of the damaged eye
He might bend his neck to allow the undamaged eye to see what the damaged eye can no longer see.
He might be able to jump - he might not.
He might feel vulnerable - so he will probably compensate by using his other senses - smell, hearing, touch, vibration.
My guess is that within a year or so, you are going to become an expert on one eyed horses.
May I suggest that you keep a diary. Make a note of the problems as and if they develop. Then ask, guess, experiment with the possible answers. Then record the results. Your experience may well become a model for others who might have to face a similar scenario.
There will be some notes somewhere in the veterinarian world of how other horses have adjusted to loss of sight. Do some research and try to find out what is available to you.
If you need help or ideas, - then post a note on the Forum - we will all try to help if we can.
Good luck and look after yourself - you've got responsibilities now.