No, cataract does not cause permanent blindness. It is a simple operation with a high rate of success that can return your vision if the cause of deterioration of vision is the cataract.
The main question is how much the cataract disrupts his day-to-day life. Driving? Watching television? If it does disrupt, then it is a good idea to perform surgery.
Something else which supports the need for surgery is the stage of the cataract. If it is quite advanced, waiting until he cannot see at all from the eye can increase the risk of surgery, as the surgery is more difficult as the stage of the cataract increases.
A change in the prescription number of the reading glasses is a natural process caused by losing strength in focusing the lens for near-vision. It is completely different from a cataract which is the process of the lens becoming cloudy.