Sounds more like constipation than encopresis at this point, but that could, of course, change if the withholding continues. I've heard strong recommendations for the book It Hurts When I Poop to help kids deal with the fear of difficult elimination. And of course, good hydration and as much fiber as you can get into his diet will help, too.
Successful potty training is not age-dependent; kids train somewhere between 18 months and 5 years, depending on an array of physical, nervous, and emotional factors. Every child is different. Some kids need more time to get the pooping thing worked through, and will withhold if they feel pressured, resulting in constipation, painful elimination, further withholding, and possible encopresis (which is a medical problem that's hard to correct). These difficulties in turn slow down the poop training, so pressuring a child before he's willing may end up lengthening the whole process, and certainly adding stress for child and parents.
I've known a number of moms who do let their kids use a diaper for pooping for however long they need it. When these children are given the time they need to sort out the sensations and their feelings about the ongoing responsibility to get to the potty on time every time, they do eventually decide they can handle it and develop a genuine willingness. At that point, training is often complete, and child-led, in a couple of days or weeks.
Reaching that point of readiness can take awhile, especially if they had become resistant to overeager parents. The latest I personally have known a child (boy) to fully day train was age 4 + 10 months, and that child is still not night trained at 7 years. But two different pediatricians convinced his parents to follow his lead, and assured them that when his body and nervous system have matured enough, he'll be night trained, too.
It sounds like you have just recently begun to insist that your son not use diapers. And that just recently he's been withholding again, and developing a fear of pooping. It would be useful for you to take note of that connection, because it suggests that what you think should work for your son is not working (and therefore cannot work for you, either).
There will probably be responses advising you not to "go back," or to "stick to your guns." There's a long tradition of that kind of thought. But the most commonly accepted medical and psychological advice on potty training is to support the child's interest, don't try to rush the process, and keep attitudes light and positive. There are also many stories on mamapedia that when moms did back off, the child was finally able to train himself.
Once it becomes a matter of "making" the child go, failure and resentment become part of the whole dynamic, and resistance is almost sure to set in.
Good luck. I hope you find an approach that results in a healthier outcome for your son (pun intended!).