Because of physiological or neurological factors, people may suffer from Blindness. Blindness is the condition of lacking visual perception. But the extent of vision loss has been divided into various scales to define blindness. You grandpa may have the partially blindness because he still has light perception.
Kid, the total blindness is the complete lack of visual light perception. Total blindness can be called NLP which means no light perception. Your grandpa only has light perception. Maybe it is not blindness. Nowadays different areas have special assistance to determine blindness. There are various governmental jurisdictions have formulated to define legal blindness. But I think you can help your grandpa by reading for him when he needed.