If you and his parents haven't already done so, use some of the children's books and videos about potty training. Be sure not to overdo it, though, because you don't want to introduce pressure. Also develop a little potty training chart, on which you place a star (or some other form of sticker) when he uses the potty successfully. The key is to be patient and supportive, not impatient, as he goes through the process. Be sure you are on the 'same page' with his parents, because this really is their area of responsibility to set the tone.