You know, I would post this on Jeff's forum. He is a dietician, and I don't recall seeing a specific answer to this question from him.
I know that in the instances where breastfeeding genuinely can't be offered to an infant that Dr. McDougall has said he doesn't know which is worse--cow's milk formula or soy formulas.
However, you are in a different circumstance. Given your child's age, I suspect the advice will be to stop all dairy, use soy milk as a condiment only for cereals, and otherwise offer the normal McDougall diet. The caloric deficit is easily enough made up with eating at the higher energy density end of the spectrum, with more juices and whole grain products than an adult might require. But I'd love to learn along with you, if you post on Jeff's board.
The perfect is the enemy of the good."--Voltaire