I had the same problem with my boy. I tried potty training him early but it didn't work, so would try again every 2 months or so. He had the same problem as your son, he would sit on the potty for a long time and then get up and go to the bathroom. I would talk to him about it and he seemed to understand but he wouldn't go in the potty. I tried diffrent things underware so he would know when he went, prizes, sitting with him, etc. Then out of the blue one day he started going in the potty. My son is now, 30 months, we are still working on his potty training but we aren't having the problem of him not going untill he gets off the potty. My advice to you would be just keep trying, don't get discouraged, and when he is ready he will get it.