What is so important about lactose in formula?

the second ingredent on the formula is lactose. the wic rep just put my son on lactose free formula because he is having loose stools from the regualr advanced formula. it didnt make sence to me that he would be lactose intolerant if i had him on regular enfamil lipil before that for 5 months. i just want to know why my son would need lactose in his diet if he wasnt lactose intolerant.