The tank looks great for this situation, Nick, but don't forget the water bowl. Maybe aim for mid to high 80s at the warm end.
The wound does look pretty grim, and I thought the worst untill I saw the close-up where you're holding him.....it might not be so bad. Don't euthanize him just yet, give a couple of days and see how he goes. He's not so small, so forget about feeding him at all for the time being. I feel pretty optimistic, you obviously care and I think you'll be able to turn things around for him. Good luck, Nick, and thanks for sharing the experience and photos with us.
Oh, and welcome to the forum!
If he does pull through, why not keep him and join our little community? You're more than welcome.