L.
Been there, done that! I have a 3 1/2 year old boy. He was completely potty trained at about 1 month before his 3rd birthday. A friend gave me the BEST advice: Get rid of the pull ups! (Except at night, at first). Also, I let him pick out "big boy underwear" (Thomas the Train), and when he had an accident in them, they went in the garbage. He quickly realized that he didn't like that, and started using the potty. Also, whenever he had an accident, we would take about 10 minutes and practice running to the potty from all different parts of the house, and pulling down pants and practice using the potty. Chaos and lots of fun. And also, when we are in a mall parking lot, football taigate, or other places where no potty is convenient, we use empty plastic water bottles, that usually accumulate in my car.
But, most importantly, don't worry -- he'll get it. How many 20 year olds do you know who aren't potty trained?? Good luck!