The part involving the Number Theory aspect was intended for the OP so he could understand the mathematical reasoning behind what the Greatest Common Divisor really means, and how it's calculated. The examples serve to show how it works, and how he can apply it to his son.
I've found that if, while you're following the examples, you say in English what you're doing, it reinforces the concept.
There is another, possibly easier to understand, method using a Matrix. Your son doesn't have to understand (yet) *why* it works, but he needs to know how to do it.
In 5th grade, I would have gotten wood over "the above"
