He is learning to tell you what he likes and doesn't like. He has had a taste of sweet juice/water. Have you tasted formula? Yuck! If I could choose, it would be juice, too. lol If you don't want to take the juice/water away from him all together, then maybe try to add some juice to the formula? That way it flavors it some? Or as a healthier option, make a "smoothie" with the formula. Add some fruit and stick it in the blender. You may need to add a touch of water for the right consistency. Just get creative. Then, when he is a year old, you won't have to worry about it anymore and can just switch to milk, etc.
Also, maybe take him to the store and let him pick out a sippy/bottle. Then, make that an only formula bottle? Just another idea to try. He is starting to want choices at this age and sometimes they really do help with "power struggles". ;)