I started by building a wooden skeleton. To get the proportions right I measured my own body and then took two-thirds of each measurement (I figure a seven-year-old boy is about two-thirds my size).
For the elbow and knee joints I used small hinges.
For neck, shoulder, hip and wrist joints I used an screw-eye in each side, and a S-hook between to get a more random natural movement.
Late in the process I had to go back to these joints to wrap them in strips of muslin. Doug was looking pretty great in rehearsals, but the metallic clicking of these joints completely ruined the illusion. Wrapping the joints allowed them to keep their free natural movement, but muffled the sound considerably.
Once all the joints were attached together, I started adding some muscle and flesh. I cut strips of quilt batting and wrapped them around each part of his body.
Once I had added enough batting to fill out the shape I started adding clothing. The costumes department was kind enough to loan me some pieces that they didn't end up using.
We tried to stage the scene so that Doug's face would be hidden as much as possible, but he still needed to have something in case the audience got a glimpse of him.
At a local costumes store, I bought the cheapest wig I could find, a cheap mask, and a pair of nylons. I attached the mask to Doug's head then pulled the nylons over his whole head. The nylons give the skin a much more natural look than the clearly plastic mask, and help smooth the transition between the mask and the rest of the head.
And here is Doug fully dressed. He looks pretty convincing in person, and has certainly startled some of the theatre staff when they walk backstage.