Feeding baby?

My son is 10 weeks old and he was feed for the first 4 weeks then I gave him Aptimil 1 after a week or so I gave him Apitimil 2 Extra Hungry because I thought he was wanting more. The problem is that he has only ever had one formed stool. He always has runny watery yellow pooh apart from that one time last week. I was told that was normal for when he was fed but not now and that I should move him back to the Aptimil 1. Have you had this experience and what was the out come? Thanks