How to get a 4 month old to take a bottle?

My son used to take bottles really well because I was expressing but now I only give him breast. He won't take the bottle at all even if it's breast milk and not formula. I'd like to be able to go out once in a blue moon without being called to come home because hes hysterical but he won't settle with anyone else and won't accept anything but boob. Any advice would be much appreciated :)