Both of the ones I was going to suggest have already been mentioned.
I think Biergarten is a great choice. It is not stuffy at all; it is a lot of fun. The food choices are very German but there is also the standard kid section of the buffet with choices such as macaroni and cheese and chicken nuggets. When I was there for my birthday this year they gave me a card signed by the staff, a gingerbread cookie, and a song.
Teppan Edo is also good. There is the interaction with the chef and the cook at the table fascinates the boys in my life. Even if your son is a picky eater the chicken and rice should be good. When I was there on my birthday this year they gave me the choice of ice cream or ginger cake, a paper chefs hat with my name (which is still being proudly displayed on top of our refrigerator), and a song.
I think you could have a really great time at either one. On my actual birthday we went to Biergarten for lunch and Teppan Edo for dinner. Biergarten is my all time favorite and I have taken my brother there (he just turned 10) and he is an extremely picky eater and we were able to find something for him to eat.