The size of the lens depends on how wide the eye is open/the eye reacting to the light. You just normally don't notice as much.
Sadly, he has lost most of his vision in the eye. He can still see light and dark and probably large expanses of colour. The sclerosis can get more dense with time. And yes, there is unfortunately a good chance that he may go blind on the other eye too, with either sclerosis or a cataract. Sorry about the bad news. It usually comes as a nasty shock, especially as there isn't anything you can do about it medically.
Don't change the layout of his cage for the moment and keep his normal daily routine. Switch to more audio than visual cues. Try not to approach him from the blind side; he won't like that! If necessary, send him a little mental picture of where you are going to take him, so he can brace himself. This will become more important for piggies with both sided sight impairment but whatever you can put in place now can make the transition easier on him later on if he is losing sight in his other eye.
Sight is not their strongest sense and a blind guinea pig's other senses will sharpen to compensate (hearing, smell and touch through the foot pads and the whiskers). Piggies that have gone blind need to rebuild the mental map of their world according to the changed information input, but once adjusted, they can move around without you noticing their impairment. At the moment, with one eye still operational he doesn't need to do it as he has got vision.
Please have the diagnosis verified by a vet. How old is Lilly? Congenital cataracts/sclerosis come on often during the second year of the life.