Depends on your sons interesses! The church itself is an outbombed ruin, with few remains, there is a great view from the top of the tower, and in the old crypt, there is an exhibition, with the story of ww 2. Maybe you could find something about the St. Nicolai on the internet, to show your son before visiting, to see if he is interessed.

I'm sure your son will enjoy the trip to the top and spectacular views over the city. The museum highlights the war years and the terrible bombing the city endured. I found it interesting if only to highlight the futility of war and it's effects on the innocent.

About an hour would do it.

Sorry for the late response; you can reach the visitor platform ONLY by an elevator !