There are so many variables to consider with your situation. First, are his poops really hard? Because if they are it may be painful for him. In which case you should try 1/2 tsp MIralax mixed with apple juice or whatever he likes to drink. Second, if you or someone else is getting upset with him every time he poops, doesn't poop, poops on a really nice carpet...etc. he is relating pooping with something bad. He needs to know that you will be patient with him and will not get mad. Seriously, he should be rewarded AT THIS POINT for anytime he goes, no matter where. (Verbal praise, high-five, balloon, m&ms, bubbles for 5 minutes, favorite book read to him...whatever he loves most) Example: "Good job going #2 (or poop)!" "I'm so proud of you! Good job. But next time let's try to go in the bathroom so Mommy doesn't have to clean this up. That's where big boys go potty. And you're a big boy. You're not a baby anymore. I love you honey. Good job going potty." The last thing I would mention as a possibility is that he may not like you in the bathroom with him. In which case you can stand just outside the bathroom or close the door and leave him alone. My old therapists would tell me to keep a schedule of when he has bowel movements so that I would know when is the best time to put him on the potty and possibly get success. Hope one of these ideas helps!

Don't use Miralax, it's poison, give him probiotics. Probiotics are natural and will help with any stomach issues he has. My son has been taking them for 3 years and is doing well, both my husband/myself take them too. As for him not using the restroom, my son finally had success with his bm's at age 9. He urinated in the potty at 3, guess it's easier to do.

@A MyAutismTeam Member
We used following ways to motivate my 3.8 years old.
At home, we have started making him wear only underpants. No Diapers. He holds himself for 2.5 hours. (we have been watching him since a month). So we learnt to catch him. ( squeezing, touching himself, restless, walking funnily when he can't control himself before peeing).
Got a new eating item (anything that he has not had much but might love it.) which he gets only after successfully using potty after peeing.
We do frequent trips to bathroom ( 30 minutes or more depending on the time of the day like after food, more frequent trips r needed). Every time, just of seating there he gets i-pad for his favorite music video. You can change this reinforcer to avoid excessive attachment. We have also done puzzles etc sitting on the potty.
Even if he's not likely to pee or potty, we do take him to potty in order to train him to lower his underpants & pull up again.
So our goals are in below order.
1. He learns to control his muscles.
2. Uses potty to pee. (Now prompts are needed).
3. Eventually tells us & goes himself to pee.
4. Once above things are mastered, we will work on letting us know about the big job.

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A MyAutis...

My daughter refuses to poop in the toilet. She also refuses to use public restrooms. She is 6

Give a tablet to play with. Tell him he can play games or watch videos while he's on the toilet.