You might be able to see him leave the plane but the gate area may have a glass partition. At other airports, people at the gate can't even see the people leave the plane and (usually) walk down a slope to the floor below for passport checks and customs. They do it this way so people won't bring sausage and fruit with them then run into the terminal to bypass customs.

Consider seeing where the Lufthansa flight departs back to Germany, go to the gate and see if there's a glass partition. If so, you may see your son and can wave. (Later, when passengers board on the next Lufthansa flight, they open the door to the glass partition and close the door leading to customs and immigration).

Then meet when your son emerges out of customs. Another way to do it is to meet at a specific place that you agree upon, such as the United Airlines ticket counter. Don't select the Delta Airlines ticket counter because that area is too big. Make sure you bring a newspaper so you can pretend you're reading it. Make sure you have an agreed upon password, like they do in spy movies. For example, you son approaches you and says "The birds fly south in the winter" and you must reply "but now that are starting to head north".

Seriously, for fun, see if you can see him at the gate through glass. Then head out of the gate area and meet near the U level (level that passengers depart customs) Starbucks. A less crowded area is Level 2, Row 5, which is the check in counters of British Airways and Lufthansa, probably towards the side (they won't want you to obstruct passengers and wait right in the middle of a walking path). Perhaps agree to meet here?