So...my oldest absolutely REFUSES to drink anything other than milk. He'll have a meltdown if I say…

So...my oldest absolutely REFUSES to drink anything other than milk. He'll have a meltdown if I say no or if I tell him there's no more. He also drinks a lot of it (like three bags in two days kind of a lot). We're getting tired of buying so much milk every week and it's getting expensive. Anyone else have/had this problem? And if so...what can I do to get him to drink something else?