To me, it doesn't look like he has a slipped tendon in the photo, & that's great! I wonder if he did just sprain something. If so, stretching it I'm guessing wouldn't be good. Just minimizing his running around if you can to help it heal.
I have never seen a bird stand one foot crossing onto the...
Do you think he might have a slipped Achilles tendon in the hock on that leg? There is info on that on the Poultry Podiatry page linked in my sig below, if it's helpful.