I wouldn't push him to much. If he gets constipated and has a painful poop, he may decide not to poop ever again. It becomes a viscious cycle, and can be very traumatic for everyone. This happened with my son at three, and there were a lot of tears. We had to medicate him to make him poop (this was not a diet issue, and he was too big to give an enema with out hurting him because he fought). And he ended up back in the diaper to poop anyway. Not saying it will come to this for you. But our doc said boys can take longer and, since it's an emotional issue, wait until he's older to pursue extreme measures. When he was four we bribed him with a Toy Story gift basket. After taking him to see the movie, I put together a fancy basket with Buzz, Woody, and smaller toys. He earned one each time he pooped on the potty, starting with the small stuff. It worked. He cried and strained and panicked on the potty, but we waved the prizes around and cheered him on. By the time he earned the biggest and mosted wanted prize, Buzz, he had gone five times on the potty. That was it, finally something he wanted enough. It was an expensive basket, but diapers are costly, too. And cleaning up a poop diaper after a 45 pound kid has gone in it is unpleasant to say the least!
When it comes to pooping, a tough love approach is not always best. The child can make themselves physically ill, and it can lead to true emotional distress. Again, maybe your son will poop on the potty later today, no big deal. But I would be careful of letting him get constipated because of other issues that can arise. good luck to you both!