It was a big challenge to potty train my son, but it eventually happened. He was 8 or 9 when he finally had a successful bm in an actual toilet. My recommendation to you is to let your child wear a diaper on the toilet, but cut a hole big enough for the bm to come out. Remember it takes time and lots of patience, your child is capable.

wait longer. And then when you potty train a while later realize you'll need to tell yourself you may need to do it a few times :) We just did our 3rd potty training with my son last year when he was 4. And I think we're finally sticking to it!
Another thing you can do to make sure he can't hold it in, is to start giving him LOTS to drink. Prunes, and dried apricots as snacks, and lots of grape juice. You may have a messy day or two with him getting back in the swing of things but you can reward him each time he does go potty by telling him what a great job he's done, hugs, whatever floats his boat.
*here's our potty training routine*: Take him to the store and let him pick some type of toy that shoots, Cheap nerf gun, suction cup bow and arrow, disk shooter etc. Pick up some of that putty stuff called FLARP. Let him also pick a special new book, and two packages of something his LOVES to eat, one bigger treat, one small (my son picked licorice, and fun size kit kats). Then go home and put the toy gun, the flarp, and the book in a container in the bathroom. This box is his special potty box. He's only allowed to play with the things while he is sitting on the potty. The toys/book cannot leave the bathroom. He gets 5 darts/disks to shoot then he needs to try going potty for two min (set a timer). After the timer is up you can ask him if he'd like to be done in the bathroom or play more. keep playing with him in there as long as he'd like to switch off playing and trying for 2 min. Repeat this process every 20 min or so while giving him loooooots to drink and you're sure to have a success at some point! Even if it's peeing right after leaving the bathroom you're getting somewhere! The flarp putty is my kids' favorite thing ever. you squish it into the container and it makes (pardon my french) fart noises. I've taught all my kids that it's totally okay to make fart noises in the bathroom but to be polite in public. They love practicing with that putty :) Everytime he pee's on the potty give him a small treat, every time he bm's give him a big treat. Make sure to never complain, or chastise when he has an accident and just cancel your appointments if you can for several days to get this all going. Good luck!

Children with ASD commonly are used to what they are used to and change is a bit difficult even taking off the diapers to do the potty process- Holding the poo is something he may have done prior and he is going to have bigger issues if he is constipated with potty training so if you must use a touch of mirolax to get that moving- They have portable potty seats available put that Not in the bathroom but put it in front of the TV or something and get him some little guy underwear to run around in- let him get wet- and feel uncomfortable a bit direct him to the potty every time. I would reward every time with a M&M candy something small and every time that will set a positive enforcement as well for him- also get him plenty of water or fluids - Ditch the diapers for a bit and see how the big boy underware goes have you son go pick them out ! GOOD LUCK ! on the Pooh mission !

I thought the general belief was your kid should be potty trained by age 3 so you're doing this at the right time. My soon to be 4 year-old daughter still hasn't graduated to porcelain throne club yet. Awareness of pee or poop is at least a start so the discomfort is normal, right? Same for the ability to control when he wants to go. My kid never gives us and indication when she takes a dump in her diapers. The best time is to put the kid on the potty first thing in morning and time it for after meals. This advice isn't foolproof as we've been playing the fool on potty training and have two kids in diapers. My son isn't autistic but he's still two.