L.A.
We were having that problem too. Then he started showing interest whenever I was drinking anything. I gave him sips of water out of my glass, a few times. He got the point. somehow, once he had had a try from a cup, the sippy was altogether easier.
Another tip is, if you have the kind that has a spill/leakproof valve in it, take it out for a few goes. That way, he can have any easy time of getting a drink and will associate the cup with drinking.