How much milk is he geting in 24 hours? Keep an eye on his intake, 20oz is the minimum he should be getting but that does include the dairy in his food so you can always increase cheese, yoghurt etc. I would keep the morning and bedtime bottle for now to keep the milk intake up, and use the sippy in the day. By the time he is 1 then he should be used to milk in a sippy and then you can transition the morning and bedtime milk to sippys.
HTH!
Laura