My son didn't get potty trained until around 3 1/2 (that was 6 months ago). If we told him to go potty, no problem. But he could never seem to figure out when to go on his own. We too were expecting another baby boy so it was even more critical to get him going. Here's basically what we did:
* Put him in training underwear. They're thicker than regular underwear so they contain accidents better. But, they're not absorbent like diapers so he could feel when he was wet.
* Reward system. He would get one M&M if we went potty on his own. (he never really got candy, so 1 m&m was a lot).
* Potty schedule. Tried to keep track of when he last went and not let it get more than a couple hours. Also made him go after he ate or drank. If he had an accident, he had to stop playing with his toys for a while. The idea being to make him understand that going potty is more important than those other things and if he forgets, those things go away for a while. I feel certain that my boy knew he needed to go potty, but didn't want to stop drawing or playing with his trains to go.
* Associate going potty with being a big boy. I don't know about little girls, but my boy is fanatical about being a big boy. Telling him something he's doing is what babies do is the best psychological warfare weapon I've ever discovered. However, I try to explain it matter of factly and not in a demeaning way. They KNOW you are disappointed when they have an accident and feel bad, so try not to make it too hard on him. Like someone else said, get him some new "big boy" underwear with his favorite cartoon character or superhero on them. Explain that he doesn't get to wear those until he can go a couple days without an accident. Hyping the big boy underwear was probably our best overall tactic.
I think at that age, little kids are still afraid to grow up. They understand they need to grow up to do bigger kid things, but they're afraid to give up things that mean security like drinking from sippy cups or using a diaper (all he's known for the last 3 years). I don't think it's a question of feeling the urge to go and knowing when to go. I think it just takes time for them to understand that growing up is okay and this is all part of it.
Also, believe me when I say that most parents of little boys go through this. I don't know why, but boys just have a much harder time with this concept than girls do. Truly the hardest part is not getting him to go potty, but dealing with your own frustration at the situation.
Good luck,
LL