"He sits in the chair."
Translation:Han sitter i stolen.
Without context, yes.
We use 'sitter i' for sitting in any sort of soft, comfy chair or similar; recliners, sofas.
While 'sitter på' is used for simple dining chairs, benches and stools, where you don't have the option to lean back and get comfortable.
I believe it matches the distinction made in English. :)
Å sitte = to sit http://www.do2learn.com/picturecards/images/imageschedule/sit_l.gif
Å sette = to put
Å sette seg = To sit down (put yourself directly translated, from standing till your butt is touching the chair or ground. http://www.fisiotic.org/essawiki/images/f/f8/Teach-me-to-sit-down.gif