Formula cow's milk or soy milk?

Hi mummies, my 2yo had episodes of diahrrea on and off(probably due to diet as well been eating whatever we eat and also caught stomach flu from cc) and doc suggested soy milk..doubt he is allgeric to cow's milk though..he's been on fm and bm since newborn..all along he is on illuma milk powder(it's very exp) and during one of his diahrrea episode we changed to Mamil Soy milk powder and he accepted the taste eventually...diahrrea also improved(not sure milk or medicine helped). Now hubby wants to change to Mamil Soy(more economical) since LO accepts. Is there any concern whether toddler is on cow's milk or soy milk formula? In terms of nutrition etc. Is Mamil Soy milk powder good as well? #advicepls #pleasehelp #1stimemom