missy2009, Hi and welcome to the Toilet Teaching board.
My suggestion would be to have your DS ask for a diaper/pull up when he needs to poop. Tell him he needs to go into the bathroom to poop. He can stand, sit on the floor, whatever he wants. As soon as he is finished have him help clean up. Do this for about a week, then have him sit on the potty with his diaper/pull up on, for another week. The next week try cutting a hole in the back/middle of the diaper/pull up and have your DS sit on the potty; most of the poop should fall through. Then after a week, try without the diaper/pull up. If you find your DS is not comfortable without a diaper/pull up, try putting a belt or something around his waist, this will help relax the muscles that are needed. I hope that helps. Please keep us updated on your DS progress.
I agree with the PP. Let your ds have a diaper! It is more important to prevent the poop withholding, esp since your ds has a history with constipation.
If the previous suggestions don't work for you, another approach is to allow your ds to ask for a diaper/pull-up to poop in for several weeks (approx 8 weeks). When he is VERY comfortable asking for a diaper to poop in, then choose a day to begin poop toilet training. On the appointed day, when he asks for a diaper, take him to the toilet instead. Since he'll have asked for a diaper, you'll know his urges are strong, so take advantage of them.
Additionally, since your ds sounds like he can withhold poop so well, a couple of days before poop toilet training, begin giving your ds daily doses of a laxative (MOM is fine) to make your ds poop softer and more difficult to withhold. Continue giving him laxatives until he is completely comfortable pooping on the toilet.
MOM is a non-stimulant laxative. It is not habit-forming (the body does not become dependent on this type of laxative), which means it is safe for long-term, daily use. If your ds can still withhold poop on one dosage of MOM, then increase the MOM until he cannot withhold poop. You just don't want to give your ds so much laxative that he gets chronic diarrhea and gets dehydrated, but otherwise, give him enough laxative for HIS body's needs. You want to keep the poop very soft, about the consistency of pudding (or softer, if your ds can still withhold at that consistency). Some other good non-stimulant laxatives are Miralax, Benefiber, and mineral oil.
Unfortunately, some kids just have stronger toileting fears than others. It's "natural," but it's not fun!
Good luck to you.