If you are driving to Berlin from Hamburg I recommend that you stop in #9 Helmstedt. I recommend that you stop to see the transit museum in this key town. We passed along this corridor in 1982 when we had to go through the border checkpoint and again 2 times in 1990 when we could just drive right through while watching soldiers ripping the structure apart. While I have been to Helmstedt on more recent occasions I haven't been to the museum but may go someday. The images are still so vivid in my mind of what I saw and experienced on those crossings from the past. Have a great trip with your son.
EDIT: another strong impression. Once you left Helmstedt you couldn't get off the highway for any reason (except the specially built, controlled petrol stations) until you reached the "Welcome to the xxx Sector" on the outskirts of Berlin . Right now I can't remember if it was the British sector or US. It was a very bleak and tense drive, especially since we'd been pulled over by an East German guard with a big gun right after leaving the Helmstedt guard station.
2nd Edit...We were actually on the A2 a more west-east approach on the 2nd of 3 transit routes between east and west. We were driving from Bielefeld to Berlin.