I am express breast feeding my son who is 3 months old. At the end of the month I'm hoping to let him go to his father's for the day (we seperated when I found out I was expecting our 2nd). Would formula milk be ok for one day as I don't have enough of a Stash in the freezer to give him my milk for the day or would this not be advised, I'm having a bit of a pickle deciding the best route. Any advance would be fantastic