M.G.
R.,
We've experienced a similar situation with one of my twin boys. They're eight months old and also exclusively breastfed. About two months ago one of them started fighting my husband when it came time to eat (they'd get two bottles of breastmilk a day while I was working). As hard as it sounds, my husband would have to "wrestle" (his words) with my son until he would eat enough. You may just have to fight the struggle. It did get easier for my husband, but he had to keep with it.
Also, as another person pointed out, when babies know the real thing is right in front of them, they often struggle to take the bottle from Mom.
Good luck!