The easiest way is to show him the formula for the area of a sphere:
Area = 4 π (radius)²
That's a good thing to discuss, because it's a good thing to remember,
and discussing it will help him remember it.
The important part to look at is the part where it says " (radius)² " .
That tells you that if you multiply the radius by (Anything), then
the area is multiplied by (Anything)² .
Since he multiplied the radius by 2, the area was multiplied by (2)² = 4 .
