Transitioning from bottle to sippy

My son is just about off bottles.  He consistently drinks his milk in a straw sippy at daycare and sometimes at home but, at home, he also at times has seemed disinterested in the sippy cup so I've resorted to giving him a bottle on those occasions.  Do you think that is a mistake?  Is it better to set the tone that milk comes in your sippy cup and if you don't want to drink it that way, then no milk?  I've been hesitant to do that because I'm trying to make sure he gets at least 12 oz of milk a day, but I wonder if it's worth it to go a couple of days with less milk until he gets the message.  Any experience with this or tips you can share?  TIA!