26 Months.. Ready to start?

DS is 26 months, and speaks very clearly. He can tell you that poopoo and peepee go in the potty and that we are not supposed to go in our diapers..

However.. whenever I set him on the potty (either the big potty with a potty seat, or one of those toddler potties) he says, "I can't!!!" He has peed in the potty successfully a few times, and we made a big deal out of it, gave him a small reward, and so on.. but he seems to have little motivation to keep doing it.

 So, my main question is this... when do I need to start being a little tougher in my approach? How long should I go with the, "I can't!" answer before I start to push the issue more so we can be done with diapers?