Change back formula

My little one drank dumex dulac from newborn, but after he turn 3 month + he always diarrhea after every feed or will diarrhea out while he is drinking formula. went to see pd the doctor gave him probiotic to stop his diarrhea but he say if diarrhea still continue, have to change to lactose free formula. we brought dumex soy formula for him to try, diarrhea still continue but not as often as when he was drinking dulac. then we switch to enfamil soy. it was better. but still diarrhea one to twice a day. so, can we change back to his original formula he was drinking which is dumex dulac?