Babies poo turning white since gradually introducing cows milk

My son will be 12 months next month so I have gradually started transitioning him to cows milk. He has had cows milk in cereal & food and cheese etc since 6 months and has been fine. At the start of this week I made him morning bottle 50/50 formula/cows milk for 4 days and then done the same with his evening bottle but some of his poos have turned white so I'm worried his digestive system is not ready. Has this happened to anyone? I have now put him 100% back on formula and will try when he's over 1.