Regression? Resistance? I Don't Know What This Is! (Long)

So you just described my kid! He had been potty trained for several months but not poop trained. We still put him in underwear though because he would not go until he was by himself in his bedroom for nap. Every day. It became routine. He also had constipation and we had to use miralax. We tried everything. Every reward you could think of, putting the little potty in his room. Then one day after a couple days of not going poo I had to give him a fairly big dose of miralax and at one point he kind of panicked because he was playing downstairs and that poo was coming out whether he wanted to or not! We ran him to the potty and he has been going poo on the toilet ever since. I don't know what happened but I really feel like your son will have something click into place one day too.
Oh yeah, my son also has speech and social delays and SPD and who knows what else. So really, I think he will get there!

My kids both needed miralax since they withheld, but both are girls and switched to underwear around age 2. I would make sure he is really regular with Miralax and pull ups and then go back to underwear. If he WANTS to wear underwear tell him he can go back to underwear when he poops in the potty without ever pooping in the pull up. I think it is much easier to wait until they are fully ready and not clean so much, so stick with the easiest route for you which to me would be pull ups (or diapers since they are cheaper and easier to removed standing up).

He's got problems in more than one area. You need to recognize that he is a special needs child. He obviously relaxes and sleep so heavily that his sphinctor muscles allow him to poop in his pants.
He is a fraternal twin so comparing him to his brother is totally unfair. You could take him to a homeopath who is specialized in children and practicing for 10 years. S/he is likely to help you with all his developmental problems.

I had the same issue with my 3 year old. He was totally on board with peeing in the potty, he even told me when he needed to go or went on his own. Poop was another story! He would purposely wait until naptime for his diaper and poop then or wait until his bedtime diaper.
I finally just started making him pee before naptime and ditched the diaper, he was able to stay dry so it worked. He then started pooping in his underware. I started by making him do the clean up. I would send him to the bathroom, make him take his clothes off and put the poop in the potty. Then he had to rinse off his clothes and clean his body in the shower. Of course during I was right there and I had to scrub the bathroom floors, sink and tub afterward. He still pooped his underware.
So I just made a time each afternoon after lunch just before his nap to make him sit on the potty. The first few times he cried and sat on the potty for about 20 min with no success. Then he waited until bedtime and pooped his diaper. I kept doing in and he started pooping in the toilet once in a while.
I also watch him closely and when he has alot of gas or seems to be in the "pooping position" (which for us means crouching down on all fours or hiding) I would run him to the potty and tell him he could get down if he made at least two big poops. He is finally getting it and even told me yesterday, " I need to go potty right now or I will poop my pants" while we were at the school.

Keep trying he will get it :o)