It doesn't make him nervous but is an obsession. I am sure. My DGS has obsessions too. He is autistic and 11.
If this helps your stepson calm himself I think the hotpads with threads to pull out is an excellent idea.
We have to find ways to help them cope with their everyday life. Obsessions are just a part of the package. We learn to channel them so they are manageable and I think you are onto something for your stepson.
fortunately my DGS has left some obsessions behind as in running to whatever catches his attention. And he is no longer obsessed with slides. now he is obsessed with printing off Wiggles pics on You tube and writing stories around them. The laptop and drawing keeps him busy for hours. He is bound and determined to make his own movie! He has always been obessed with movies and tv shows and the behind the scenes info as in what studio makes that program and who the sponsors are and what ads are shown on there. He could one day become a director who knows??