I am sorry to hear that your grandpa got cataract. If a person get cataract, he may suffered from clouded, blurred or dim vision etc. However, most people with cataract are not painful, but there are really seldom people that feel uncomfortable. Therefore, I suggest you should take your grandpa to the hospital.
I think the most important thing for you to do now is to send your grandpa to the hospital. People with cataract are usually painless, but they will feel hard to see things clearly gradually, especially in the evening. Therefore, To help your grandpa's vision, I suggest you to send him to the hospital as soon as possible.