Is the story of how he became a shut-in as or more interesting than the rest of the story in itself? If so, you can start with that if you want, or flashback once you present the premise. You can have half the novel be the backstory, if the backstory is super interesting. Probably not as exposition, though?
Is it not as interesting, but you feel the reader needs to understand it for the next bits to make any sense at all? Well, show or tell just enough that the reader knows what's up, make it as interesting as possible, and then move on to the actual story.
Is it neither necessary nor interesting yet, but will be when certain events come across that parallel it? Well, the reader doesn't need to be told about it yet-- you can note how another person acting shut-iny parallels his life when it comes up, or briefly note why he's acting a certain way when it would otherwise be strange for someone to act in that manner.
If it's never necessary to the story to understand his past, you can also just leave out the exposition of his past almost entirely, and just have the little hints of his past be something to piece together if the reader is interested in doing so. This is especially true if you aren't let inside his head much/at all. Then the mystery of it can be interesting, even if the details themselves aren't too interesting.
TL;DR: think about as a reader how interesting you'd find it if you did information on the backstory in different ways.
