Yup, this is also an option.. Since he's already 2.

Give him the bottle / cup and everytime he drinks from the cup, compliment him, clever, good job, big boy etc.

I dunno why, but I kinda recoiled when I imagined milk and orange juice. It just doesn't sound like a good idea.. My son's formula is a combination of both the choc flavour + plain or sometimes honey flavored milk powder. He seems to like it. Does your child like chocolate? Maybe can try giving him the choc flavoured milk powder..