G.B.
If he likes computer games it might help to sit him on the potty and give him a hand held game. Have him sit there until he poops. Then give him a reward. We did 2 M&M's for poop and 1 for pee or jelly beans, same ratio.
It helps them to feel special and want to do it again. Just make sure he's not getting them any other time during the day or it takes away the "special".
If he is autistic or has some other issue he may never be the same as your kids or at their level. The mom sounds like a case for sure. Too caring because she knows there's a problem and she is in denial.
I'm sure he's going to go through some testing eventually because the teachers will know there is something wrong.
Pull ups are to teach a child to pull up and push down their underwear and protecting your furniture and floors. I hate when people say they aren't useful. If a person has a lot of extra time to clean up poop and pee off their stuff then by all means, let the kid do it anywhere they want.
If you don't want to hand wash poop out of his underwear and his clothes then go rent a rug doctor with upholstery tools to clean your floors and furniture then by all means, tell them to supply you with pull ups.
I used pull ups and am eternally grateful I didn't have to clean poop off my carpet.
Just try to reinforce his good behavior and help him feel good about going, not bad about having an accident. He may also have a biological issue.
We put diapers on clients that were not in control of their bowels when I worked with developmentally disabled children AND adults.
If this child has a physical disability that effects his bowels and his ability to manage them himself then the school system cannot keep him from attending school, they have to supply someone to assist him with toileting. Handicapped children get to go to school too.
I'd also sit him down as soon as he comes in and manage the chips or anything else they give to him. If he can eat them all then have nothing left he might eat other food as he gets hungry. This way he'll have better food in his body.
When I eat regular potato chips I almost always get diarrhea. It's mostly grease and chewed up chips. Sorry. But if I eat other food I do much better.
I suggest you start slowly and work him into eating better food and then work on the potty training too.