First off, howdy and welcome to the forum
Honestly, the best person to give you a good answer to the future care of the eye and whether or not it would have to be removed would have to be between you and your vet. We can do a lot of speculating but not much more.
The thing that concerns me most is that he is only 21 months old and is riding "well". That is a very young age to start a horse undersaddle and will predispose him to arthritis as he ages. Personally, if I bought that horse, I would want to turn him out to pasture until he was at least 3 to give him time to finish growing so that I wouldn't be inflicting any more damage on his growing skeleton.