Question 722866
Hello neighbor. My seventh grade advanced math students were in your son's seat last year, so I may be able to help. Let's get right to the issue.
We are calculating the area of a rectangle. Draw a rectangle on the paper - remember all corners are "right" angles (90 degrees), just like the wall. A rectangle has two sets of equal sides, length and width.  Now label one side l for length and the other side w for width.
The area is usually designated by the letter A. The formula for the area of a rectangle is
(1) A = l*w, where * means multiply the variable,l, times the variable, w.
Now look at (1). In order to get area, A, we need l and w. This is ALWAYS true, as you found online. 
The unique characteristic of a square is the all sides are EQUAL, so l = w. Then we have, for a square with equal sides, s
(2) A = l*w or
(3) A = s*s
So you see, the only reason you need one side for a square is because the length and width are the same number! The square is a special case of a rectangle.
Now to get your son to solve (3) when the area is 36 sqcm, ask him "what number when multiplied by itself equals 36?"
You should have
(4) s*s = 36 or
(5) 6*6 = 36 or
(6) s = 6
You may want to give your son the MSM drill that I give my students to learn all the squares of the counting numbers, 1 to 25. Most know up to 12 from previous grades, but I find it advantageous to know up to 25. MSM means Mad Square Minute.
While we're at it, you may want the area of figures. I recommend Wikipedia.
For area you always multiply two numbers, thus square cm from cm*cm.
Square:
A = s*s
Rectangle:
A = l*w
Parallelogram:
A = b*h, where b is length of the base and h is the perpendicular distance between b and its parallel.
Trapezoid:
A = (1/2)*(b1 + b2)*h, where b1 and b2 are the lengths otf thee parallel sides and h is the perpendicular distance beteen them.
Rhombus:
A = b*h, see parallelogram
PS Live near Buffalo, NY