I'm afraid your son is conceptually wrong.
He forgot the statement that " x, y, z are any integers"
8x + 4y - 5z is divisible by 7
Therefore,
16x + 8y - 10z is divisible by 7
16x + 8y - 10z - 7(x+y+z) is divisible by 7
i.e. 9x + y - 17z is divisible by 7
i.e. 9x + y - 17z +21z is divisible by 7
i.e. 9x +y + 4z is divisible by 7
I consider this question is difficult for most of the Form 2 students. Only the top 5% in Maths can tackle it.