If he will let you, do it for him and talk him through it. Explain carefully WHY he is supposed to wipe. Over and over until he really is ready. But I'm not speaking from personal experience - my son has wiping issues, but not at the same level you describe. For a long time his only wiping was by "auto wipe" - the wiping that naturally happens from your underwear, as evidenced by the stains. He just stubbornly refused to do more. My threat was to do it for him - he was already 8 by that time and didn't want me to.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).