I'm sure you googled this, and noticed much of the concern of poop smearing is centered around preschoolers, even toddlers. But I did find this link that might be helpful. It talks about people on the autism spectrum smearing poop as a reaction to boredom lack of stimulation, or anxiety especially with those with OCD. Has the COVID crisis caused him to be sequestered at home with little stimulation?
https://autismawarenesscentre.com/it-happens-fecal-smearing-in-individuals-with-asd/
Have you consulted a therapist for him? Is he functioning mainstream in other ways?