Barn cat with a funky eye

My boyfriend rents his grandparents barn as a kind of warehouse, there is a kitty that lives there, and one of his eyes is a bit odd looking. He’s 3ish, neutered male, about 12lbs, appears to be in good health. He uses a litter box, and is well behaved.
One of his eyes is normal, the other is darkly clouded over. From my boyfriend’s account, it’s been like that his entire life. It doesn’t seem to bother him but I am curious as to what it may be.
For those who may be concerned, I am absolutely in love with him and we are going to transition him into a house cat. Eventually he’ll be fully indoors, and hopefully he’ll have a long happy life after!