Probably not the right one, since it looks very metal, but *[EON Kid][1]*? It features a young boy in a desert, finds a robot arm, sticks his hand in it. I have the part in the first episode where he puts his arm in embedded below:

http://youtu.be/b9n4OKyVQUc?t=6m23s

  [1]: https://en.wikipedia.org/wiki/Eon_Kid