Hi, you have a car, that is great to get around. I guess you like to get away from the base a bit, get some other culture and views. I donīt know where you like to stay budget wise, but here comes a treat, especially for your son.
Go to Bacharach, a small town at the Rhine, close to the Loreley. Overlooking the village, the Rhine and the hills, there is an old castle, 12th century. This is now a German Youth Hostel, open all year and has also nice family rooms. It is good budget, like a huge museum, great fun for children, good to get on a Rhine boat trip and close to Bingen, Mainz, Koblenz, and the Mosel River.
Search for Burg Stahleck on wikipedia, it has images and the link to the place.