I wouldn't do what you did

Because you've darkened him and he's wearing black clothes meaning he just looks like one black blob.
I'd start by colour balancing the scene, as it feels a bit blue with a green tint, were fluorescent lights used?
Then if you want it completely white, you could clip the whites to a point, and then cut out the rest of the scene using some manual rotoscoping.
TBH, I think the ungraded shot looks fine, except for colour balance, if you want it could use a minor contrast enchance.
If you use a threshold tool to create a mask, you can quite easily separate him from the background, some of the stuff on the right is too dark though, but again, you could just crop that if everything else is white.

Then just crop the sides.
And yeah, a png is pretty bad to work with.