Same as you went for his head...
See what you think - 10mins work.
I messed up the forehead a little (sorry) on the first so redid for the second...
Used your two pictures, copied the first onto the second as a layer.
Changed the layer transparency to 50%
Moved the layer so the head is in the right place
Added a mask (transparent)
Used a brush, circle fade, set to around 2 on size, replicating at 1 (brush slider) to paint in the head.
Skimmed close to neck to the chin left side.
Right side skimmed to jawline (matched beneath so neck still looks good).
Background area probably would need to be faded/blurred - might show on a print, but on the screen here it seems to match well.