P.M.
It takes littles a while to connect the dots between sensations and results. It's possible your son isn't quite there yet, but he could be close.
Here's a wonderful, informative website you might find helpful. It gives a few variations on"readiness" checklists, plus tips on various training strategies, the best ages to start them, and the advantages and disadvantages of each approach: http://www.parentingscience.com/toilet-training-readiness...
Good luck. I hope you and your son have a positive adventure. If at any point it just becomes discouraging or infuriating for either of you, there is no harm in stopping and waiting for a little more maturity. Once a child is truly ready, training can happen in a week, or even a day, with minimal accidents.