We ate at the Biergarten at our last trip, but we had enough people in our group for us the fill the table.
Besides what Selket mentioned, one thing we do for places like that is to try to make reservations for the end of the eating period if possible. That way, as people leave, they don't get replaced and instead of getting more crowded as time goes on, it gets less busy.
I do have to say that the Biergarten is very noisy while the band is playing. If your child with autism likes sensory input, it would be a good place to go. And, however noisy he is, he would probably not be that much bother to others. If he doesn't react well to noisy environments, it might not be a good place for him.