supplementing formula issues

So.. so. Is 4m and I'm starting to give him one bottle of formula a day and pump so I'll have some saved for the weekends bc right now I'm pumping at work for him to eat on the next day.. well I started it tonight and he would not take the formula.. we got Enfamil infant gas ease... how did you ladies start formula?! I only want to do this for like 2 weeks until I have my supply back up !