I have asperger syndrome, but when I get to much stress I regress and start wetting/soiling my pants. I dont know if that is the case for the most of the autists, but it may be a possible thing to check out since many with autismspectrum disorders have different kind of regression when they get to much stress.
What follows is an answer I recently provided for another question that I believe is also applicable in this case. Many children have problem with day-time enuresis. The issue is often the case that they really aren’t aware of their need to use the toilet, so this is no indication at all of your son being lazy. It’s something that he needs help with so that he’s better able to recognize when he has to go. Here is the answer I provided previously:
Whether your son has autism or not, this problem is entirely treatable. It is often sufficient to arrange a relatively simple differential reinforcement procedure for this. Identify some of your son’s favorite activities and set them aside for him to earn for keeping “clean pants.” You’ll want to start by rewarding him for periods of time that he can handle easily. For example, if it’s a once-a-day sort of thing, perhaps a special treat can be earned at lunch for going the morning without soiling, a special treat at dinner for the afternoon, and one at bed for the evening. If he’s able to understand, make sure you explain the contingency to him. Also, it can be helpful to arrange, at first, regular intervals to check (e.g., every 15 minutes). This can help him become aware of what he’s doing and allows you to catch it relatively soon after he soils. If he’s having a very hard time, there are monitors that you can purchase for enuresis that will sound a little alarm if he becomes wet. Often these alone are enough to work for treatment as the child becomes aware of the sensations that precede soiling and is then able to avoid it.