Shelley and Rose,your thoughts are my thoughts. I don't know if the rehabber I'd already been in contact with would care for this little guy or not, since they told me they would most likely have to put the other one down. They obviously weren't willing to give that guy a home and he was a lively little rascal. I don't know how to contact anyone else, except maybe via Internet.
I also have wondered if he's been like this for more than a year or so, since his wounds are so well healed over. If so, then no his shell is not growing back - at least not quickly.
Turtles do survive the winter by burrowing into the mud. I hope he can still maintain body temp. On cold nights early in the season (like tonight is going to be for us - 44 degrees) they sometimes get into water which stays much warmer than the air. Do you think just being close to the ground also keeps them warmer? A big part of me is saying let nature take care of him, he's made it this far. At least he has both hind feet, which I think really pushes him more than his front legs pull. But of course, balance on steep slopes could be affected by loss of a front leg and speed, too, is lost (not that they zoom around much anyway).
Keep positive thoughts and prayers going his way. It's a big God, he's got time for the little creatures too.