Dedicate an entire weekend to "pooper-duty" and go cold-turkey from the pullups and diapers. He'll use em as long as you put him in them. Let him pick out 2 to 3 packs of real underwear with his favorite characters, and tell him to try his best to keep them clean and dry by letting you know when he needs to go. To help prevent accidents, use a timer or potty watch (www.onestepahead.com) and set it for 30 to 60 minute intervals, and take him to the bathroom on a schedule, until he gets the hang of keeping the pants clean and dry.
If for some reason you must leave the house, DO NOT use diapers or pullups. Instead use rubber training pants (Gerber makes these, and they can be found in the toddler/baby section at Target). They are regular cotton undies that are padded and covered with leakproof plastic. They still leak, but it will buy you time to get him something clean and dry if you're out and about. The key is, you want him to experience the uncomfortableness that comes with an accident. You want them to find their reward in being clean not in just getting a treat for doing learned behavior.
He'll have a few accidents to start, but after experiencing the mess, he'll quickly get the routine and by the end of the weekend, he'll be trained.
If you'd like, you can reward him after an entire weekend of clean undies with a treat of some sort.
This works. It's hard work, but if you pay your dues over a weekend, it's well worth the chasing him around. We did this with my youngest when nothing else worked, and it was THE way to go.