It affects the amount of sugar in your blood vessels in your eyes. It usually levels out but your eyes need to checked leakage now and regularly which can lead to blindness.
High Blood Sugar (BG) can cause many problems with eyes, heart, kidneys. loss of sight is one thing that uncontrolled diabetes can cause.
There are lots of other reasons for eye sight problems, but high BG can cause vision problem and can lead to total blindness. He should see an Opthamologist ASAP.