I have a two year old (Oct. 5 2013) that started holding his poop in about October of 2015. Up to now, he has allowed himself to go on average once every seven days in a 6 month span. Our family doctor put it this way- "give him as much of whatever kind of oil that makes him go every day." In a 6 day period, we had 14 teaspoons of mineral oil and 3 ex-lax squares in this little guy!!!!!!!! before he finally did go. And no, he IS NOT constipated. This is our seventh child, and I don't think we have ever encountered anything so frustrating and helpless. Likewise we have tried seemingly everything, the stern approach, bribing, candy, celebrating when he finally goes, ect. ect. Still, just walking toward the potty gets him bawling every single time. (By the way, we were never even attempting to train, he just quit going in his pamper, so we thought maybe he doesn't like the feel and would rather go potty)_We have resorted to castor oil, which used to get the best response in the fifth or sixth day. Now, even that seems to have no effect anymore, and I am worried sick that he is damaging his colon. What is our next move?
Your frustration and concern are evident, and it doesn't sound like the advice given by your family physician was very helpful. If they can't do better, time for another opinion. Your next move is to get a consult for a pediatrician who has some experience in managing chronic constipation.
While voluntary stool holding might not sound like constipation, technically, it is, even if you're doing everything you can to avoid it.
Constipation generally is defined by the hard nature of the stool, the pain associated with its passage, or the failure to pass three stools per week.
Encopresis is the fecal soiling that often occurs with constipation. You don't mention this, and it's early for this diagnosis, but entertaining it would do no harm.
There is some reason your child refuses to go to the bathroom. It could be medical (a bowel innervation problem), physical (he had/has painful bowel movements - due to an anal fissure or other - and wants to avoid them as much as possible), psychological (he's a bit young, one would think, but they do have psyches), or a combination of things. This is the beginning of an investigation and training period (for both the child and the parents) that will take many months, so please know that you're in for this problem for the long haul. The good news is, it's early yet, and getting proper diagnosis and treatment can save you years of dealing with this.
In the meantime, the basic approach is elimination of pain with defecation, increasing fluids including osmotically active ones (pear and prune juice), fiber (with fluids; some fiber without fluids can bind people instead of help), and, if the child has a lot of dairy intake, e.g. cheese (which can be constipating), decreasing dairy intake may be helpful.
Note: I am not a psychologist. But, this sounds fear motivated. Maybe at some point in the past he had a painful constipation / stool. Maybe sitting over the yawning void of a toilet scares him. Maybe cold water splashed him once and that startled him. Who knows.
But the best way to overcome fear is practice. As stated in user1751825's excellent breakdown, fiber and fruit are your friends. Miralax is good. Over time, as nature forces him to eventually go again and again, he will (probably) get over it.
Again: not a medical or psychiatric professional. Consult your doctor, and don't be afraid to consult a child psychologist if issue persists.
Waiting 6 days I know seems awful, but they do tend to get over it, and it doesn't seem to cause any lasting harm.
I would steer clear of harsh chemical laxatives if you can possibly avoid them. These should really be a last resort.
For ours we did on occasions use Parachoc, which is a chocolate flavoured paraffin based laxative, but we were careful to avoid giving it too often.
Avoid the stern approach. This just makes things worse. For whatever reason he has apparently decided he's frightened of the potty, so you don't want to do anything that will make him more anxious.
Try to make sure he's well hydrated, with water only. Give him plenty of fruit, and also you can try adding some psyllium husk to his meals.
Hydration is super-important. Particularly if you increase the fibre in his diet. Fibre can absorb more water, so you kind of need to over-compensate, or excess fibre can actually make things worse.