Don't give him the cup...if he needs a drink do it before he goes to bed. He's playing a good old game of get mum and dad wrapped round my little finger and winning hands down
Why don't you try getting your toddler to drink the milk whilst you read a story and then when you leave the room it's lights out with no distraction. If he/she has not finished the milk by the time you have read the story then it is obviously not needed and is removed when you leave.
I totally agree with Ellie on this one. Your son is really young so there is a chance this can be nipped in the bud. My son is 4 this month and once he's in bed all he does is call after me and his dad. We give him a kiss and a cuddle and that's it. After about 5 minutes of calling us he gets fed up and goes to sleep.