My big story has multiple IC’s, and I’d like to tell one of their stories, as while his story deeply intersects with the MC’s, there’s a lot going on with him that the MC doesn’t know about.
But I’m having trouble figuring out his storyform. Do I use his big story IC throughline? I’m having trouble making it work using this IC now as MC.
Or do I create a whole new storyform for what he’s up to?
Suggestions appreciated. 
You create a whole new storyform for him. Let him shine in his own light as it were.
Maybe, switching mc and ic for that segment…something done in many romances, at least it seemed that way as I was reading the story.
As Good As It Gets has two storyforms that aren’t precisely what you’re imagining but darn close. As described on the Dramatica.com site:
As Good As It Gets has two stories in it–the Romance Story and the Neighbors Story. In the Romance Story (shown below) Melvin Udall is the Main Character and Carol Connelly is his Influence Character. In the Neighbors Story Melvin plays Influence Character to Simon Bishop’s Main Character. SYNOPSIS: “A single mother/waitress, a misanthropic author, and a gay artist form an unlikely friendship after the artist is in an accident.”
Never seen that one, I’ll take a look. Thanks.