My DS1 was FF fed from birth and like you I put cows milk on his weetbix and in cooking and so on.
From 12mths we went straight onto cows milk with no problems. I just made sure he had enough 'rougage' in his diet so he didn't get constipated. In fact his poo improved on the cows milk On the formula it was a funny army green color and very smelly
Things like Kiwi Fruit, Grapes and also plenty of water and no juice other than at breakfast helped.