Select flag using magnetic lasso, copy, paste onto other image. Right click in layers palette, convert to smart object. Resize as desired. See attachment one.
To have it behind him just resize then add a layer mask, select a black brush and paint him - big strokes so you can see him, don't be accurate go way over the edges. Then grab a white brush and paint the flag back.
Total time it took me is about 2 minutes, that's why it looks like a 3 year old did it 
HOSTED PHOTO
please log in to view hosted photos in full size.
HOSTED PHOTO
please log in to view hosted photos in full size.