Well, my 27 year old mare has pretty severe Moon Blindness (or ERU, "Equine Recurrent Uveitis") in both eyes (best guess, 85% of her total vision is gone) and I'd love it if that were the case! Lol Let me know how that goes for you! :)
It is a degenerative disease though. Each "attack" is followed by a small or large decrease in vision. Basically the attack triggers are as different as the horse's are. For my mare, her trigger is getting chilled so I go out of my way to make sure she's never allowed to get chilled. Another thing that helps most ERU horses is to be flymasked with a mask that has UV protection whenever it's sunny out. Some horses need to be flymasked all the time for it to really help, but others, like my mare, only need it when it's really sunny.
Unfortunately, I'm going to have to say that if your guy is 2 and already diagnosed with ERU, he will probably eventually lose sight in that eye. He might be 20 before it all goes but it's basically impossible to completely avoid attacks even with 100% perfect care. It's not a reflection on the caretaker at all, it's just the nature of the disease. ERU is the absolute worst. :(
Maybe you're already seem this but this article helped me understand this disease so much better than I did before. It's still confusing to me but it's less than it was. Equine Recurrent Uveitis: Information for the Horse Owner