Advice on baby milk and cows milk

I have a one year old that was on aptamil from birth. Then at ten months it started to disagree with him and was swapped to lactose free aptamil which he was fine on. I was advised to try him back on normal to see if it was a phase so he's now on stage 3 aptamil which seems to be disagreeing with him as he's got diareah again. I'm not convinced he's lactose intolerant as he's okay with yogurts and good with milk in. He will not take cows milk at all. Anyone else? I'm going out of my mind