K.C.
Are you the one offering him the bottles or is someone else doing it? Make sure someone else is offering him the bottles for awhile and doing it at a time when you are not around (ideally not even in the house). If he can see, hear or smell you, he's going to reject the bottle and just cry for you.
Pump so that he's getting breastmilk and not formula in the bottle at first. Then, gradually mix it so it's a combo of both (heavier on the breast milk in the beginning, eventually moving towards more formula).
Another thought, since he is already nine months, is skip the bottle and just give him a sippy cup. Some babies like regular ones, others prefer the kind with the straw. He might take to it better from a cup since it's very different and not at all like your breast.
Also, just know that he may go on a bit of a milk strike and reject it, but eventually he will get thirsty and he will drink from the bottle or cup if you aren't offering him the breast anymore. For your own comfort, I'd recommend cutting one feeding every week or so. If you cut him off completely, you will end up engorged and miserable.