I do not want to alarm you unnecessarily but my son also did not want to use a sippy cup and wanted someone to hold it for him. It had nothing to do with laziness. He is now 3.5 and we have learned that he is on the spectrum for Autism. Because he was my first child, I didn't recognize the signs I was getting. He's in programming now, but wish I could have started him a lot sooner.
I only say this because I'm learning that occupational things are frequently an issue with autistic children. If your son is on track developmentally and you don't have any concerns, just disregard my thoughts. I'm sure your son will start using that sippy cup when he's thirsty enough and no one is there to do it for him. However, if you have started having any concerns at all, you may want to have him screened for developmental issues. Your school district can get you started.