Please do NOT release this turtle into the wild. There are many reasons, one being that it just might be (and probably is) illegal. PLEASE read this article for a good explanation as to why it is a VERY BAD idea. www.anapsid.org/release.html. This site also has a link to herp rescues by state. Perhaps you have one near you.
I know you said your vet said it's OK but he is misguided in this. Your turtle would not have natural immunities to organisms that wild turtles have nor would they to ones that he might have. It's a very delicate balance of nature thing.
Perhaps there is somebody on the board who lives close enough to you to take him in. If you let us know what state you are in that would help.