Any parent here starts their baby on soy milk? At what age do you start? Pros & cons as compared to formula milk?

Normal soy milk milk powder? Those sold in supermarkets. Actually, without any allergies to cow's milk, babies can oso take soy milk powder rite? My 7mth old is still hving diarrhea even though he is on lactose free formula milk.. Previously subsided, but when i start back on his usual milk, diarrhea came back again & tiz time, e anti diarrhea meds does not wrk well alrdy..