what's this mouse?

We live in Southern California (Ventura Co.) and we found a very small mouse in our backyard. We tried to identify it using this website, but to no avail. It looks like the Western Harvest Mouse, but according to this site, it's not possible. We thought he was a baby mouse at first because he was so small, but he has a lot of hair, so we guessed he couldn't be a baby. The mouse is all gray, including his hairless tail. He's about 1 1/2 inches long, 3 inches w/his tail. He usually sits on his hind legs and we just noticed today he jumps. What is it?