HELP!

I need some help! My sweet little angel has not been so sweet to me lately. He has come to a stage where he is testing the limits and pushing my buttons and he is winning. He has gotten into the habit of talking in a ugly voice to me and not listening to what I tell him to do. I have been trying to stop milk at bedtime as he will down an entire sippy cup just before going to sleep. He doesn't have trouble wetting the bed but I am concerned about the health of his teeth. So, we have been really talking about sugar bugs and cavities lately. So I have explained to him about not having milk at bedtime or he will get cavities. Last night, mind you he had no nap and was up late, he wanted milk while trying to go to sleep. Just constantly asking for milk, whining, and flailing all around the bed. I try to calmly tell him no but he knows if he bugs long enough I'll give in because I feel he will not EVER quit. So I get him a CUP of milk. He takes one little sip and goes directly to sleep. So- do I start off just giving him one little sip or torture us both by holding off until I can't take the bugging anymore?

We have lots of episodes like this and I seem to be loosing the battle. I try to stay tough because I don't want him to think I'll always give in, but I pretty much always give in!! Whenever he asks me if he can do something and I say no he just keeps asking and asking and asking and asking and asking and asking. Get my point?? No one ever told me a three year old would run right over me!!