Have you consulted a therapist for him? Is he functioning mainstream in other ways?
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?