Introducing combination feeding to 4 mo old

Does anyone have any advice on starting to combination feed an older baby?

He's 4 months old and has been EBF. He's willing to take a bottle of breastmilk most of the time. I'm a just-enough producer and have a tiny stash for emergencies. I need another option for when he spends the day with his father.

I've tried giving him a few bottles of formula in the last couple of days since getting the thumbs up from his ped but he gets mad when he realizes it's not what he's used to. I know I could mix in breastmilk but since I don't have a lot I'm afraid of wasting if he keeps rejecting.

Should I just keep at it? Thanks in advance.