So, by that reckoning if my son stands on a wall so that he is above me I'm still higher than him because I'm taller?
No, you need a scale. For example, many people determine Everest to be the highest mountain above sea level. But you can use a different base level, the centre of the Earth perhaps which would give you a different mountain, the tallest from the base of the mountain to the top, which would give you another.
When we talk about the highest mountain, we are usually referring to the highest above sea level because people are rarely below sea level (and you couldn't climb the ones under water!). But it is equally right to say that there are higher mountains when measuring from the base to the tip.
Your son is at a higher point, compared to you at that time. However, when you are using a uniform base point you will be taller. Everest is a higher mountain than the one in the ocean, when measured by above sea level.