Great save thus far!

WC garters are sometimes stressed for a few days by being held captive, and in this case, you have the injury as a stressor too.

The tank looks fine to me, but a little bare. You might want to take some paper towels and shred them into strips, then sprinkle the strips around on the floor of the tank, to simulate substrate. Having something to burrow in will make him feel more secure.

Good luck with him/her!