Have you tried the potty seat inserts that go on the the regular potty seat? I took my son who was 3 and in diapers to Kmart - a liesurely shopping trip, just the two of us, no rushing, special just for him. We picked out a potty seat (they are cushie and he picked out a Sponge Bob one). While we were there, we also visited the boys underwear section. He saw all the cool character undies and picked some out that he really liked. Then I told him that I would buy him the Spiderman undies that he really liked once he had 2 dry days in a row, but the undies had to stay at the store until he could do it. I then picked up a set of training undies - those really thick cotton underwear that were boring blue. He had a fit, but I explained that he needed to practice. That these were practice undies and I would get the Spiderman ones later. He had played soccer before and understood that you have to practice to make a goal, so I said that it was just like soccer, but wouldn't take as long. I made the return trip to Kmart with him 3 days later to get his Spiderman undies. Yes, that fast!
I think the reason it worked so quickly was because he was invested in it. HE had made the decision to go on the potty, with his special potty seat, and, honestly, I think he was afraid someone else was going to get his Spiderman undies if he didn't do some hard core practice.
One thing to remember... Daytime dryness does not mean they are ready for nighttime undies. My son (now 5 1/2) wore a night diaper for 2 more years. Only pee, but nighttime dryness isn't something they can control. It's a body maturity thing and for boys this can take until they are 5 or even 7.