If I had to guess I would imagine it would have to do with how young he was separated from his mother. I've often heard of kittens separated young having suckling issues, although this is the first time I've heard of eye balls lol. Not all suckling cases are because of an early separation, but a good portion are. I had to be a surrogate for Cloud at 4 weeks of age, and he often tries suckling/nibbling my nose.
Suckling and kneading are instinctual behaviors and often the very first thing a kitten does when out of their mother's womb. They are usually fed by mama kitty until 4-6 weeks of age where she will begin to start weaning them. Sometimes mother cats will let them nurse longer, usually for comfort and reassurance. Nursing is a very special and tender time for mothers and kittens, and it fills a kitten with nothing but happy feelings.
A lot of kittens that are taken away from mother cat too early will suckle, it is usually for comfort. The cat sees you as its mother, so he is approaching you with the same awe that a kitten has towards a mother cat. Typically a cat will suckle their human when they are feeling happy, safe, and content. See, it's a compliment
Being around you brings out those same feelings that the kittens have around mother cat, and he is instinctually responding to those feelings by suckling. Usually kneading is also accompanied with this, but I didn't see mention of it in your post.
Most cats that suckle like this will continue this behavior for the rest of their lives. I wouldn't consider the eyeballs an ideal suckling place at all. This could cause you some serious problems. But the suckling is an instinctual thing that you won't be able to stop. I would recommend trying to transfer you kitties suckling ways onto a new object. It needs to be something non toxic and safe with nothing on it that can cause the cat to hurt itself while suckling (like plastic beads). It can be a cat toy or non furry stuffed animal. Make sure to rub it on your arms that way it carries your scent. It is you that your cat finds comfort in, so your scent being on the object is important. Whenever the kitty starts suckling you give him the object. It will take time and patience doing this transition. But it is a compromise that will eventually leave both you and your cat MUCH happier