What happened? Was he bitten? When did the injury occur? Is it bleeding?
Here's a link on the subject of shell injuries:
http://www.redearslider.com/shell_conditions.html
This turtle needs antibiotics administered by a vet - topical ointments or washes won't be sufficient.
On the link I provided above, you'll find another link to a page on treatment of minor shell injuries. You can use the advice on there for interim maintenance until you can get your turtle to the vet's, but your turtle's shell problem is not minor - the minor shell treatment is merely a stopgap to try to control further infection until you can see the vet.
Good luck. Your poor turtle is probably in pain. I hope your vet can see him right away. Please keep us posted on how it goes.