That's how I taught Matt. We worked 5 hours a day, six days a week for about 4 weeks. Assuming that a player thinks of his head and spine as an axis, the shoulders rotate around that axis. The hips follow, and on the forward swing, the arms swing across the body around that axis. Essentially, posture dictates the shape of the swing, and that in turn dictates the shape of the shot.
Good posture allows that extension to happen.
The bad news is that he'd never played before, but the good news is that he had no bad habits. There are plenty of books out about golf stretches. Even bulky people with limited flexibility can play as long as they stretch. My shoulders tend to stay open and the ball takes a good flight but always right. As much as I try to align correctly it doesn't seem to work.