I would first ask, what sort of cups have you tried?
My daughter didn't take well to the hard-top cups (playtex ones) to begin with, so we got her some of the Nuby cups with the soft flexible tops. Also, has he mastered the "turning up" aspect of drinking from a cup? The tilting his head back might be half the problem.
Let me show you the ones we used in progression. Copy and paste the links to view the items. All fo these should be available at your local wal-mart of from www.baby-wise.com. If not, try the website the pictures are from.
The cups we started with can be seen here: http://us.st11.yimg.com/us.st.yimg.com/I/thebabybungalow_...#
The short stubby design with 2 handles helps them learn to tip their heads up without havign to tip terrifically far to get a sip.
If he's figured out tipping his head up and the nipple shape is the problem, try these: http://us.st11.yimg.com/us.st.yimg.com/I/thebabybungalow_...#
The nipple on them are very similar to a regular bottle.
After she got the tipping of the head down, she figured out that the cups above could be turned on their sides and pushed into the carpet, causeing the soft nipples to leak without barrier. SOOO...we moved on to the Munchkin cups, which are actually labeled as "disposibles" but we re-use them and love them for around the house and the beach and such where we don't have to worry if she loses them.
http://www.munchkin.com/userfiles/Image/travelware%20spil...
That's a picture of them, they can be found at the Munchkin website http://www.munchkin.com/products/detail.html?pID=121&...
My local BJ's is where I got mine. And they came as part of a set with dishes and lids as well. I highly recommend the dishes since they hold just the right amount of food for a child that age.
Playtex cups can be purchased from your local wal-mart as well, http://www.baby-wise.com/images/10-2680-01b.jpg shows a picture of the Insulated ones I recommend. They're a little heavier but they keep your kid's drink cooler longer.
All of this being said...I DO NOT recommend the Gerber line of cups. The valve they use and the way they work is VERY difficult to keep the mouthpiece clean and you'll find them developing nasty mold quickly since they can't be cleaned well in a dishwasher or by hand.
Make sure you're showing him how to use the cup and that it's cool to use it. My hubby sat in the floor with the cup and would drink a few sips and smack his lips like he enjoyed it, making sure he tilted his head waaay back to drive the point home. My daughter would go over to participate and he'd hold the cup up above her head until she looked up, then bring it down to her lips until she looked down. Then he'd drink a little and offer it to her. We did this for about 3 days before she had the idea down.
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I wish you the best of luck and perserverence. It's tough to deal with a screaming child, but sometimes tough love is the best they can get. Lord knows he won't want to be sipping from a bottle when he's 10. :)
Take care!