Can you try mixing it with breast milk? The taste will obviously be different, so it might be worth trying to do 1/4 cow's milk, 3/4 breast milk and see how that goes and then gradually increasing the amount if you're able to express. This is often what's done with FF babies as the taste of cow's milk is very different than formula, and of course, the same goes for breast milk. I imagine if he's not used to having a bottle or cup, he's probably wanting the closeness that BF provides, rather than just to fill his tummy. Then it might be helpful to have someone else offer it rather than mummy as obvious BF will be preferable to a cup or bottle.