I think this is pretty typical for his age and he probably gets a big kick out of throwing it down, having you pick it up and give it to him, and him throwing it back down again. This probably won't be a big issue for your new daycare provider -- she'll most likely expect it to happend -- but you can always do one of two things that I can think of right now:
#1 When he throws his cup to the floor, look at him and animatedly say, "Uh-oh, looks like you are done with your drink. All gone." And then pick up the cup and take it away. This will stop making it fun for him to throw his cup on the floor right away.
#2 Sit with him while he is eating and, whenever you notice him get ready throw his cup, put your hand on the cup and gently push it back down next to his plate and tell him, "We don't throw cups."
If you are consistent with any one of these approaches, he will get the message and the game will cease being any fun.
Hope this helps.