My DS had terrible trouble changing over to cows milk. It gave him the runs when we swapped. Our GP said most people make the change to cows milk too soon and that 12 months isn't a magical age, especially for FF bubs as they don't get the antibodies from Mum. We took it really slowly and mixed his formula with cows milk til he was just under 2. He's fine with it now. My advice would be go as slow as your little one needs you to, his poos or vomits will soon let you know if he's coping!