Bottles

According to "The Book" Callum should have given up bottles at 12 months, he still has them now at almost 20 months. We've been told they are his dummy and we should throw the bottles out. He doesn't carry them around in his mouth sucking on them, when he's finished his drink he puts the bottle down (or in the sink - love this!) and off he goes. That doesn't sound like a dummy to me. He will drink from a sippy cup, but given a choice, he prefers the bottle. He doesn't seem to drink as much from a sippy cup as he does with a bottle and living in the tropics, the amount of water he has is important. Is it really a problem if he has bottles for a few months yet?