hi T., i wouldnt sweat it at all. my son was 3 1/2 when he finally did it! i had tried everything under the sun, i knew he knew what to do, he just didnt want to yet. he actually did the same thing, went on the potty a few times, i was using a reward chart, i thought we had it. then i had my second baby and he then lost interest for months. we still tried, encouraged him all the time, etc...then one day he was just ready, and that was it, totally trained, both day and night, almost right away, no issues. he still has an accident once in a blue moon, but they are only because he gets too busy and waits until the last second to go. almost every accident has actually been in the bathroom, he just couldnt get his pants undone in time. (actually, there is something on the sesame street website where elmo talks about going on the potty and "feeling it" and he started echoing him and that seemed to help.) he has been trained around 9 months now, and he has had maybe 8 or 10 accidents since the first day he decided to go on the toilet.
my girlfriends son is almost 5 and is still in diapers, its a huge issue now, it has become a power struggle between them, you dont want that. just be light and very very positive when he does go. get a couple of books or videos about it, but just let them be another part of what he reads/watches, dont pressure it. also, i let him see his friends using the potty, i think that helped a lot. and dont you dare feel like a failure! this is just one of many times that your kid will be different one way or another, they are all just different, thats all. its hard, but dont fall into that trap of comparing kids. hth, D.