There is no medical treatment for pterygia, so once they start to impinge on the cornea, it is advisable to have them removed surgically. This is not a major procedure and is usually very successful in stopping the growth of the pterygium and preserving eyesight. You were thinking your son's condition might be a cataract. A cataract is a dimming of the transparency of the lens of the eye. There are quite a few different types, some occurring in children, but usually it's a condition of older adults.
Diabetics tend to get cataracts early, as do people on long term cortisone treatment.
Cataracts also can only be treated with surgery, and if this is not done they can produce complete blindness. We used to allow cataracts to get quite dense before suggesting surgery, in part because the older surgical procedures were easier with a dense cataract, and in part because the very thick eyeglasses required to compensate for the removal of the lens were a pain to wear. Now the procedures are much improved, and a tiny artificial lens can be implanted in the space left by the natural lens. The lenses can be chosen to correct for any nearsightedness that the person might have had before surgery. My wife, who had been very nearsighted her entire life, saw perfectly at a distance without glasses after her cataracts were removed.