Half of the Side A gates and half of the Side B gates are at one security station and you can actuallly take the trams from one gate cluster to the other.
The other halves are at the other security station.
Depending on exactly what gates you and your son come in on, you may be able to meet at a gate. Take the tram to the main terminal normally. But wait for the second set of doors to open and get off on that side. (Correction follows) When you get off there are four tram tracks side by side with platforms in between, let's call them #1, #2, #3, and #4. If you got off #1 or #2 then go around the ends of the tracks to the platform between #3 and #4.
Gates numbered 1 through 59 are at one end and gates 60 on up are at the other end.
The main terminal is a rectangle that you can think of as a football field. The tram stations are at the goal posts with security in the middles of the end zones. Sides A and B are the side grandstands respectively.
One of the concourses with stores is just outside the security for the lower numbered gates. It is a good place to meet and the Magical Express area is just downstairs from there. (Using the football analogy this open area corresponds to the area goal line out to the 20 yard line.) Or you could meet in the food court (50 yard line)
From anywhere in the main terminal you can all go to the Magical Express area together.