My ds is 14 mths and still on 3 bottles a day. Two are formula and one is rice milk, which he loves but hasn't got much fat in it - so probably not really filling. Still eats lots too. Haven't changed to cup yet as haven't even thought about it i guess. He drinks a lot of water too from a sippy.
If he loves it and is still eating a good amount of solids, i would leave it for now. Im sure when he is ready he will start showing signs...