Advice please

Pediatrician advised me that my son needs to be completely off the bottle and on cows milk by his 15 month appointment. He’s currently down to 3 bottles a day and I’m working on dropping down to 2 within the next week or so.

My problem is, he won’t drink cows milk when I offer it to him... I offer it to him in his straw sippy cup with every meal and snack. He’s had maybe two sips in the past three days and spits it out and throws the cup on the floor.

I’m not sure how to get him to drink it..? The pediatrician said to not offer it mixed with formula as we’re trying to wean from the bottle and to offer with meals.

Do I just keep persisting and he’ll get it eventually?