You really will need to address this in a PPT for his IEP. Children with ASD, even high functioning forms of ASD, are typically late toilet train. Even if they train when a typical child would train, there are nearly always frequent regressions through the years when it comes to toileting. Regressions can occur with stress, medical issues, anxiety, while learning new skills, while learning multiple new skills, or a combination of reasons.
My 10 year old has always, always hated using the school and other public restrooms. It's a sensory nightmare for her to use them. On the few occasions that she'll use one, she goes to the nurse's office and uses the private bathroom in there. She's also had kind teachers offer to let her use the one in the teacher's lounge. The majority of the time she just pees before school and as soon as she gets home. She withholds poop unless we're home, and I won't even get into the poop issues.
So. I would be encouraging, but I wouldn't make it a power struggle or a "YOU MUST! THERE'S SO LITTLE TIME!" sort of thing because it won't be as drastic as you're imagining. The fact is that like typical children he HAS TO make the choice to do particular toileting issues, and even then it might still be a problem for him. He's really not choosing to be like this.
This is partially a transition issue for him, but I would bet my left butt cheek that it's also a sensory issue. Remember that he knows kindergarten is coming... so he's beginning that transition process now. As part of his IEP I would get him into summer session this summer so that he's already getting exposed to the school he'll be attending and getting familiar with the surroundings before it's crowded and packed on the first day of the new school year. He'll get a jump start on some of the academics, which he's going to need to reinforce what he's learned in preschool and will be learning in the coming year. It will also help him with the transition process, hopefully ease some anxiety, and then let him focus and ease anxiety about transitioning out of diapers.
The biggest tip is not to give him an expiration date on the diapers/Pull Ups. See if you can find some compromises, like "on weekends let's try real underwear when we're at home during the day." Start really small and use positive reinforcement with whatever is his currency.