I assume you're using PS, so instead of cutting him out of another photo and pasting him on the background, try this:
Take a photo of the background on a tripod without him or the chair in the frame, then with the camera in the same place on the tripod take the shot again with him on the chair in the shot.
Then, open both the photos in PS, placing the one with him and the chair on the upper layer. Auto-Align the photos, then simply use the eraser on the upper layer to erase the chair from the shot and the background (without the chair) will show through. That way, the lighting will match the guy in the shot and the background and you won't get that jagged outlined around him. Give it a go!