How easy is it to photoshop someones head onto another persons body without anyone noticing?

It is easy if you are good with Photoshop, hard if not! Best to just try and see how it looks. I have some things that look absolutely 100% and others that look dreadful!

Depending on your brother's skill at recognizing light-and-shadow patterns, it might work - if he doesn't take time to closely examine it. It is not necessarily "easy," at all, but is doable with the higher-end graphics packages, like Adobe Illustrator or Photoshop. It also depends on how big the final image is to be. Jagged edges that couldn't be seen on a 1-by-2 or 2-by-3 photo could be quite visible on an 8x10. Mostly, though, the believe-ability of the image will determine the succes of the prank.

"Without anybody noticing"? Yes, it is very difficult to alter an image without anyone noticing that it has been edited. You can do it as it easy but it does not necessarily mean that it will come out perfect.