My BB was in the same situation before-He refused to drink since around 4 months old, not even 1 OZ. What I did was to feed him while he just fell asleep.
I had visited the doctor then and the vitamins helped a bit. Maybe it is because of the teething.
The situation became better when he was 5 months old. Now, 6 months old, I have just changed to formula S26-No2 and added some congee. He drinks less than before every meal and not much in total. Hope that it becomes better later. Seems that after he has a well sleep and feel hungry the he will drink well.