My son is having Isomil soy milk mixed with S26, but he doesn't have such problem. He took Isomil for a week for start, but also didn't have such problem. You could check with your doctor or give him some 開奶茶. My mom gives my son 開奶茶 when he doesn't 便便 & it works.
My son had eczema, doctor said he maybe allergic to lactose & suggested us to change to soy milk. His eczema recovered after having soy milk, only a little bit on his head once in a while now.