D.W.
Look for a sippy cup with valves - most have them and regulate the flow of the liquid based upon how hard the child sucks.
Some are better than others, so I'd recommend buying a few different ones and seeing which works.
Some are made for younger children (6 months +).
Don't be surprised if you're giving both a sippy cup and a bottle if he refuses the cup. Neither of our kids really took to the cup until they didn't have a choice between it and the bottle.
Good luck.