Help!

Someone please tell me how to supplement formula with breast milk. My lil man has been ebf for 3 months and I'm going back to work. I thought he would easy (I know crazy) to take a bottle but he's not. I can't practice with him because he will not take a bottle from me and really hard to take a bottle from anyone to be honest. I have tried 3 types of bottles and nipples; he is just preferring boobie over it.