Is he really busy kid who doesn't want to stop playing to go? My 2nd was like this. This might sound crazy to some, but it worked for us. For this, we used the Wii because he LOVES to play it and we limit it to only special occasions at our house. But you could use the same strategy with a favorite DVD or TV show, or anything he likes to do that has him sitting still for a few minutes.
We put the little potty in front of the Wii. And he was only allowed to play Wii while he sat on the potty. And every 2 hrs or so, he would get to play Wii for 10 minutes while he sat. And sometimes (just by luck at first) he would go while he was there - which of course we all made a big deal about as a reward. After a week or so, once he was going pee on the potty more consistently, we changed the rules just slightly so that he had to sit on the potty first, then he could play Wii for 10 minutes. After a week of that, he only got to play Wii for 10 minutes if there was actually something in the potty after he sat on it. Once he really had it down and was telling us he had to go, THEN we moved the little potty into the bathroom and phased out the Wii.
No, it wasn't a 3-day method of potty training - I tried that and it was a disaster. It was about a month-long potty training method. But, it worked. Oh, and during this time, he was in underpants at home and pullups when we were out, because we couldn't stay home for an entire month while he figured it out.