Hi,
You can introduce curd/yogurt to him. He may not be liking the taste of the milk. You can consider changing the milk brand. May be another milk brand do the trick. Also, since he is 6 months now, start giving him solids and calcium rich foods so that even if he is not drinking enough milk, his calcium needs are met. But make sure all the foods that you are giving the baby are nicely mashed and minced so that the baby can swallow it easily.
Here's a link below that gives a detailed list of foods that are calcium rich apart from milk.
https://www.thepresentfinder.co.uk/dept/teenage-girls-presents-gift-ideas_d01172.htm