It could be hyperfocus:
but… I’m not sure hyperfocus pinpoints one interest.
As you said, it’s a symptom of autism, but that doesn’t necessarily mean he meets the criteria for diagnosis.
You could certainly use this interest as an educational opportunity in all sorts of ways too.
ADDitude Community Moderator, Author & Mentor on Parenting ADHD, Mom to teen w/ ADHD, LDs, and autism