You cannot convert a prescription to what the acuity is. The prescription is merely a measurement of how his eye focuses images onto the retina. This is the focusing power needed in a lens to aid his eye to focus an image perfectly onto the retina.
How this image, once focused, is received in the retina and sent to the brain and processed as vision is measured by the acuity (this is the 20/20, etc.).
So, a prescription can focus the image; but if you have a bad retina or optic nerve or brain, it will not translate into good vision.
So a glass prescription cannot be translated into acuity. You will just have to call the office and ask what his best corrected acuity is.
Hope this helps.