Answer:
Step-by-step explanation:
Considering that the question is a fun riddle, I will give a suitable fun answer.
The question says that the son goes to school, so we can assume that the boy is a young child. So, he is probably short.
So, while getting down the stairs, it is easy for him to press the button in the elevator to reach the ground floor as the button for ground floor is the on the bottom of the switch panel.
However, since he is short he cannot reach the button for the 10th floor after returning from school. He can only reach up to the button for the 5th floor. So, he travels up to the 5th floor by the elevator and from there he takes the stairs to reach the 10th floor.