Baby refusing bottle?

Chantelle • Trying for #1 since Oct 2013

My son is recently refusing his bottle. He can go about 7-8 hours without drinking. His top two teeth recently broke through so I'm hoping it's teething but once he finally opens his mouth enough to drink, he will usually polish it off. It's just the initial getting him to take it.

Side note: he is almost ten months and has no interest in holding his bottle. Can sip water from his cup without issue

Side note #2, I am mixing formula with freezer stash of breastmilk, just about done my stash and it is 75% formula now. Not sure if it's the taste?

Suggestions please!!!