S.E.
Hi. My first questions is "why does he have a choice?" I have a 5-year-old and a 3-year-old. Whenever I wanted them to start something new, I'd make it exciting. If that didn't work, I'd use a reward system. And sometimes, I had to get serious. In looking back over my parenting, I can see where I was trying to make everything easy for my little ones. The problems with that are many but mainly I see how I was running myself ragged and they were like little princesses on a throne! So, sometimes I just go back to the "because I said so." adage and walk away.
So, about the sippy cup...he or she will get thirsty enough to drink from it eventually (IF that's the only choice). If bottles are still in use, forget it. Who wouldn't rather drink from a bottle? One more way; if you want to ease into it, give water only in the bottle but yummy juice or milk from the sippy.
Good luck!