My oldest boy (of 3) just turned 4 last week. He self-potty trained at 2 1/2 but would not poop in the potty. When my youngest was born 4 months ago (so, when my son was about 3 1/2), he started pooping in the potty, but reluctantly. Now, he has completely abandoned the potty and will instead hold in his bowels to where he is in pain and will not go (even if offered a diaper). I was concerned with him as a baby because he seemed to have many sensory issues. For example, he could never walk around without shoes on, would hate to be in water, and didn’t like tags in his shirt. He still seems to have many of these issues, and I am unsure if his potty issue is related and how to handle it.
Since you have definite sensory concerns related to your son, I would consult a pediatric occupational therapist (your local early intervention 3-5 program should be able to provide this) who can evaluate your son and determine whether he would benefit from therapy to help lessen some of his sensory challenges. The toilet training could indeed be related to sensory issues and although I am not an expert on that subject, I found this website that does discuss toilet training children with sensory concerns and this is an excerpt from that website from an article by Nancy Konigsberg MA OTR/L.
I am going to touch briefly on the subject of toilet training. This can be an issue regardless of whether or not the child has sensory issues. For many children, physical security is a necessity. Remember how you used to wrap your legs around a chair when sitting. Well, you did that to stabilize and feel secure. A small bottom seated over a big space can be unnerving. Make sure that you use a toilet seat that fits your child. Also, the noise of a flushing toilet can be upsetting to a sensory child. If you flush after they leave the bathroom, they will develop a habit of not flushing. Find a strategy that prepares your child for the flush, whether it is singing to themselves, focusing on something else in the bathroom or counting down the flush. Bowel movements are often problematic. I’m not going to say a lot right now. I will mention one trick that is very good. Many children, especially those who fall in the spectrum disorder category, are scared to let go of their bowel movement. For those who will poop in their pull-ups but not in the toilet, here’s a little trick. Start by sitting your child on the toilet while wearing the pull-up. Get them used to being on the toilet. Once they tolerate sitting, the next step is to cut a hole in the back of the pull-up. Make it big enough so that your child can poop and it will all go into the toilet. After they are successful, show them their accomplishment and praise them. Having them go while wearing the pull-up gives them the security they need.
I would also consult with your pediatrician to make sure you son is not constipated and finding bowel movements to be painful which contributes to him holding them.