Since Sis wants to go, hope you are all able to go as a group. I do agree that Biergarten usually works for picky eaters, because there are a number of items where any sauce is on the side and it's the sauces seem to make food issues for many people.
For your son, would it help to explain to him up front that it is a buffet, that he can choose or not choose whatever he likes, and then you and the rest of the family simply ignore what he does or does not eat? (And then - out of his hearing - STRONGLY reinforce this with the rest of the family, so that nobody picks at him and sets up a fight?). So that if all he eats is pretzel bread (like big soft pretzels, but in short straight sticks rather than criss-crossed like a pretzel) everybody will be OK with it? It may or may not lead him to eat anything, but at least it will not detract from the rest of the family's fun. At age 10 he can go cruise around the buffet and make his own choices; it's not like a toddler where you bring them a plate and then they reject everything on it. His plate, his choices, his tummy.