Hello B.,
have you tried a different brand of soymilk? they also have oat milk, almond milk, rice milk all enriched w/ calcium and vitamin D.
google calcium alternatives, milk alternatives, etc.
here is a link that can help you:
USDA National Nutrient Database for Standard Reference
Reports by Single Nutrients
http://www.ars.usda.gov/Main/docs.htm?docid=15869
search by food
http://www.ars.usda.gov/main/site_main.htm?modecode=12354500
also look at nomilk.com it will give you ideas. you don't have to agree with everthing they say there but it will have good info/links.
while you're at it, make sure he gets enough magnesium and vitamin D. Oats are great for bones because it has silica which helps the calcium to be absorbed by the bones.
Good luck! and your son will be fine. ~Carmen~