I thought usually the withdrawal was more of just a craving. If you hadn't been giving him soy before, and now he's getting a lot of soy, through the soy milk, I'd say it's possible that it's a reaction to the soy. Try keeping him off the dairy and give him rice milk instead, and see if it helps. The correlation between milk and soy protein intolerance is pretty high. I give my son one of the Children's Pepto (bubble gum flavor) every day as a calcium supplement (doctor recommended). It soothes his stomach too.
Kathy, mother of 3, wife of 1. My new recipe blog: www.kathysrecipebox.wordpress.com (no longer searchable by allergen, but at least it doesn't have a virus!)