Doesn't drink all the milk unless in a bottle

Gi

He'd drink his entire cup of milk in a straw cup at a restaurant in 1 sitting, but when we're at home, he won't finish it. He usually has his sippy cup of water throughout the day. I can't replace it with a sippy cup of MILK throughout the day... he wouldn't have his water at all for the day and the milk would go bad sitting out all day. Is he just not ready yet to get off the bottle?