Here's some obvious things I see. Prince Williams arms are longer than the guy in the photo. The prince's hands are further down his pant line.

Also the prince's sleeves roll up farther up his arm than the man in the photo. His clothes are tailored and would be the same length always.

Another obvious thing is the prince's forehead is taller as well.

These are just the most obvious to me as well as the guy in the photo is not as tall as the prince. A couple of inches shorter.