The best starter cups are the ones with a spout shaped slightly like a nipple. Gerber makes a good one for that in-between stage from bottle to cup. And always put your child's favorite drink in the cup. I never put juice in a bottle anymore, only in his cup. Having to choose between a bottle of formula or a yummy cup of cold juice, my son chose the cup.
Children stick to the bottle only for security reasons.
So once again, if asked to choose, most children will pick the better tasting. Clap when he holds the cup, even if he doesn't take a sip. Show the child that you are proud he's considering it. If the child drinks, give him a cookie, a sticker, a favorite toy, whatever your child loves. Some children hold onto their bottles because they love the secure feeling of mom or dad holding them and feeding them.