Kids do, frequently, have a hard time with pooping in the toilet, even if peeing comes easily. For many children, it's helpful to treat it as an entirely different stage of training. It's not uncommon for kids to continue to ask for a diaper for pooping for several months after pee training is a success. I've seen this happen in several families, and it does not appear to confuse the child. They just know what will work for them.
For anyone raised in a culture that uses squat toilets, you'll find a general agreement that squatting helps the process along, and kids often tend to squat in their diapers.
The urges and sensations for peeing and pooping are different, sometimes even painful, and can be complicated by diarrhea or constipation, or swing from one to the other. It usually takes a lot longer to poop than to pee (my grandson gets pins and needles in his legs from the pressure of the toilet seat by the time he's done.) Some children are very regular, some might poop four times one day and then skip two days, and never know what to expect. So it's often much harder, and this isn't your son's fault.
And some kids develop fear of the potty, or the sensations, or are terrified when they see a "part of themselves" in the potty and see it flushed away. This fear is hard to reason away in a young child, but usually they outgrow it when their cognitive functions are better developed. Parents can sometimes help this by understanding that the fear is real, and perhaps cheerfully encouraging the child to flush when the parent has a bowel movement. And explanation of "when food goes in, poop must come out" can be useful.
Most developmental specialists suggest that children be allowed to approach this challenge at their own speed, with parents acting as coaches, personal assistants, and cheerleaders. Kids do take on the challenge when they are matured enough physically and emotionally. Then it becomes a positive process for the child, and they are as proud as they are with any other new skill. Many parents report that any stage of training can take from one day to a couple of weeks when the child is confident he can do it.
I was raised at a time when children were routinely trained before two – except, of course, for the "failures." It was often a forceful and punitive process for kids that young, and I have no fewer than 4 people (3 of them men) in my immediate circle of friends who have needed counseling to recover from the trauma and rage of their bathroom training. I've also heard many senior citizens report that their potty training, if they remember it, was a harsh and intimidating experience. (I'm curious about the outcomes of parenting techniques, so I tend to ask some pretty odd question.)
I'm grateful on behalf of children everywhere that opinions have shifted toward a more respectful and humane process.