I was also fed soymilk as a baby, any other types made me hive.
And I don't THINK there's any proven fact about antipsychotics hurting the babies or anything, I think it's just definetly passes through the breastmilk and there's lack of testing (for obvious reasons) to show what developments or harm it could cause the baby.
Have you tried soymilk with the baby? I would not breastfeed with this med. I would not take the chance regardless of the dr. I used soymilk with my oldest, who was allergic to everything. Good luck