It’s really great that you’re seeking help and talking openly about this! Encopresis is too often shameful and kids suffer when parents don’t speak up.
There’s lots of conflicting advice out there and I strongly recommend you find pediatric autism specialist if as all possible. A generalist MD or therapist is not likely to be knowledgable about this topic.
This website, despite the sales pitch, has good information and a detailed manual. It might be worth reading. https://www.encopresis.com/encopresis-manual/
Editing to add that interoception may be part of the issue. People with ASD often have more distant perceptions of internal bodily sensations than allistics, so your son may only perceive his need to defecate at a 1/10 where another kid might feel it at a 7/10. Teaching body check ins and mindfulness can be helpful, but is tricky with young kids.