First, don't worry. It's not like your kid is going to be wearing diapers to his college graduation. So relax. If you're uptight, he'll be uptight. Act like it doesn't bother you.
1. Make a big deal about buying big boy underpants. Go out together and let him pick the ones he wants.
2. Put him in underpants the second you get home. Explain to him he needs to listen to his body and put his pee pee and pooh in the potty. (or whatever you call it)It also helps if you point out that all of his friends now use the potty.
3. If he has an accident, involve him in the clean-up. Don't act like its a punishment. It's just part of being a big boy now. Be very cool. Take him to your bathtub and have him undress himself and bathe himself to get all of the pooh off. It's distateful, even for a kid. So he'll learn fairly quickly that it's easier to drop a duece in the toilet.
4. Be consistent. Don't go back to diapers or pull-ups no matter how much he pleads. It's just no longer an option. Not even for bedtime. This is important.
Good luck. You'll be fine.