Pooping in the potty comes months after peeing for many kids. The child has to be ready, and confident enough to relax. Sadly, we can't just tell them to relax, especially if they have felt pressured to "Relax, hurry up, and succeed, already!" That whole complex of mixed messages, and the sense that they are disappointing their parents but are still confused or anxious about how to let go on the potty, is simply a set-up for failure.
Plus, emotional tension or excitement can mess with the action of the automatic muscles in the bowel, just as tension or excitement can interfere with the automatic of the diaphragm when we breathe.
So you can do your best to time his potty trips to match his natural rhythms. Many children will poop shortly after they eat. If the urge is strong enough, he may be able to go on the unfamiliar potty seat. But he may not be able to for quite a while yet, and that's not his fault. He will get there when he's able.
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...
Once children reach the point of withholding a bowel movement because of emotional pressures, they are at risk of developing constipation, pain, and the much more serious condition called ENCOPRESIS. You can google that term, and "potty training, withoulding bowel movement" for lots more information and ideas. One of the main points made on most sites, however, is to ease off on the pressure and give the child a chance to say when she's ready.