he may be totally blind in that eye. having a blind, or partly blind betta isn't that different from having a betta who's not blind at all. i, personally, own a blind white HMPK, named Theodore(Theo, for short). It's tough at first, getting used to having a betta with a handicap like that. it was lots of trial and error. i found that i couldn't use rocks on the bottom of his tank like with my others. that way, he can still eat all his pellets, and not waste any that he missed out on. i also never change his decor, it's always his big pink plant, and smaller plant with the wide flowers he can rest on. and, i trained him to eat from a black, floating ring.
but, that's just what I do with Theo. i suggest it to anyone who thinks they have a blind betta, because it's easy on me, and on Theo. he knows where to run to if he gets scared, doesn't have to worry about getting 'lost', and knows where to get food when it's feeding time.