S.C.
Have you tried holding him when he drinks the sippy? That seems to be the trick for my kids. I didn't let my kids run around with a bottle, I always held them when they drank it. I think they associated that cuddling with drinking. Once I got them used to drinking out of a sippy while being held, then they would run around and get drinks every so often. I still have to remind the little one (she's 17 mos).
Sometimes it's all about the KIND of cup too. My oldest hated cups with soft spouts. She would only drink out of the Playtex cups with the valve. My youngest hated hard spout cups and would only use the Gerber ones with the softer spout. Neither liked the Nuby cups.