You can probably go cold turkey IF you use the right sippy cup. Really, its possible, and easy most of the time. Again, its all about the cup.
Nuk brand. Its got a soft spout (much softer than the leading brands, who tout flexibility but I cant agree. Hard as a rock compared to the Nuk one. Problem is, its HARD to find!! Target, Walmart, BabiesRus, none carry it. I went online (drugstore.com stocks it) after seeing my nephew's cup. They had him off of bottles, cold turkey, at 9mos old. He never cared because this cup is so nice on their mouths.
And the best part is that the cup is NOT made in China, but rather Germany. So you know that there's no lead. Its a little pricey due to that, as the workers are actually being paid livable wages, but I was glad to support non-China products. The cups last a LONG time, they are made well, not cheaply.
We get by with only 3 cups because they clean like a dream.
I swear, if you use this cup I bet he'll convert relatively easily.