I know this problem well as I spent a decade as a middle school theatre teacher and started from scratch so I had to recruit all my students. For a year or two our ratio of girls to boys was 10:1. I received a piece of advice from a colleague at another middle school ('Improve your production values... Good tech will hook some boys into backstage work, and from there they will go onstage,') followed it, and ended up with a 3:1 ration for the rest of the time I was at the school.
However, I think it's time for a bigger conversation here- and Laura Russo's response gets to the heart of it. If theatre is expanded to the elementary grades, much of the stigma disappears because kids grow up with the art form, and it is 'just what they do.' I am an elementary drama teacher. By the time my students get to middle school and high school, they are surprised if boys are NOT participating in theatre and other arts classes.
Elementary school isn't just the domain for music and visual arts. We need to change this.