DMCA notice procedure is near the bottom.
It's not a sure thing, but reporting it to Tumblr might get it pulled.
You could make an even better case if you can find other stolen stories in the blog. Take an early, unique line from a few of the posted stories, put it in quotes, and search for it with google.
If you find a hit on another site, make a note of it with the URL and the URL on Tumblr. If possible, contact the author and link them to the DMCA instructions. The more people who report it, the better.
Check a few like this. If you can locate several stories by different authors posted to the blog, then it reinforces your case that he's grabbing these and claiming them as his own. Could possibly get his account deleted wholesale.