My baby is 5 and a half months old and he refuses to take expressed breast milk as well as formula. What should I do?

Does the baby still breast feed ? Is this a first attempt to not breast feed ?

If you are just introducing a bottle, you could try having someone else offer the bottle (with you out of the room, out of sight, and so he can not hear you). You could try offering it when he is very tired. You could breastfeed first to satisfy his initial hunger and try switching to the bottle part way through a feeding. You could offer when he is very hungry, hold him very close (skin to skin) and be sure that the milk is already flowing from the nipple just as if you were nursing him. You can try using a nursing necklace to distract him. Hold him and let him play with a brightly colored, chunky necklace you wear and offer the bottle when his attention is focused on the necklace.

You may find that you can skip bottles altogether. Many babies will start using a sippy cup around this age. That can be another option to try.