Your nephew is myopic. Usually myopic patients tend to have retinal degenerations in the periphery of the retina. You should explain him to report if he notices following symptoms:
: Sudden onset appearance of moving black spots in front of eye
2. Flashes of light : Apparent bursts of light seen in the peripheral vision
in the absence of any such a real light
3. Fall of curtain in front of the eye ( 1/2 or full)
This is very important to diagnose earliest if some problem arises.Please explain him.
Now, your ophthalmologist must have seen some lesions / weak areas which does not need any treatment. But I will suggest you to visit some retina specialist so that lesions can be categorized better.
Hope this helps you.