L.K.
My son is 12 months old and on whole milk now, too. I still keep powdered formula on hand, though. If I know I'm going to be out (especially somewhere hot like the park), I take the formula and bottles/sippy cups of water so I can mix it as needed. When we are at home, I use the whole milk. It is also helpful to have the formula on hand in case we are running low on the milk. That way I always have a backup. When we're at home, I don't mind putting milk back in the fridge as soon as my son loses interest in drinking it. I will throw it out if it gets forgotten and sits out a while.