Hi Janine! and welcome to little Jake too

I b/f and have never used formula, so I don't know how to help you to get him to take it. But personally, if it were me, and it's only 1 milk feed that's going to be missed, I'd get him onto solids before you go back to work, so the carer can give him solids and water, and not worry about formula. If it was more than one feed, I'd persevere with expressing or formula. But you can always give him lots of extra booby at the end of the day, and top him up before you leave him.