Someone mentioned having him clean up his accidents. It is unsafe for a young child to be messing around with poop accidents. (Which is why you see the daycare workers donning latex gloves when dealing with soiled diapers.)
Urine is at least safe for them to handle without the possibility of spreading germs such as e-coli. I still question the wisdom of that punitive approach, because I've seen firsthand the way negative associations with potty training can backfire, but at least it isn't physically hazardous to thier health.
Generally, I try to avoid bribes because they can backfire. (For example, a kid that gets a treat for reading gets the idea that reading is work, not reading because it's enjoyable. Humans (not just kids) are funny that way.) BUT, in the case of potty-training, bribery doesn't seem to have much of a downside. There's not much satisfaction derived from going in the potty, other than the "I can do it!" aspect. Too often, the novelty of being able to do it doesn't last long enough for the habit to sink in, and sometimes kids backslide.
The most important thing is not to get into a power struggle. (Sounds like you haven't yet, or your post would sound much more desperate.) If you get into a situation where you're having to coerce him into sitting and he's arguing or fighting you... THAT's what I mean by "power struggle." Find SOME way out before you get to that point, because that's when you REALLY run into problems. Generally, I never want to go backwards once starting on this kind of thing, but if you start to lose your patience and get frustrated... it *may* be better to go back to diapers for a short period. That's NOT the OPTIMAL solution, but it sure beats "power struggles" which can drag out potty training from months to 1-2 years. Seriously.
Worst case scenario that can result if you "go negative", an example of what NOT to do that happened to a friend of mine:
When he was about 4-5, his father got tired of bedwetting accidents and hung up a clothesline outside to dry the underwear, telling the boy that all his friends in the neighborhood would know that he was wetting the bed. My friend was so embarrassed that he NEVER wet the bed again. Instead, he forced himself to never get into a deep sleep state, wake up every few hours during the night, and never sleep past 5am or so. So he wasn't wetting his bed, but he developed a sleep disorder that was a much bigger issue. He was sleep-deprived through all his school years into adulthood, until he finally sought treatment. There was some long-standing resentment to his dad over that... Yikes.
Not to freak you out, just reinforcing the importance of keeping it a positive experience.