:Take him out of the pond and bring him inside. Place him in a rubbermaid or somewhere you can dry dock him until the vet can see him. Also give him a warm basking light (not hot), soak him in warm water for a little while every several hours or so. Keeping the shell/wound dry and warm will help heal it and keep it from becoming infected furter. Other people will respond to your post with some good ideas shortly probley
Actually Stunt -- this is pretty good advice
Hopefully the turtle is septic yet and you taking him to vet tomorrow so bring him in and take him tomorrow. Good luck and keep us posted.