There is an injured garter snake in my yard?

Yesterday after a big rainstorm I noticed a little garter snake on my front walkway. He didn't look injured, but he wasn't moving. I left him there hoping he would go away but this morning he was still there. I more carefully inspected him and realized he was only moving the front half of his body. His back was motionless like he was paralyzed. Now I feel really bad for him and I don't know what to do. I picked him up by his tail and moved him to some wetter gr and out of the way of any animals. It looks like his back is slightly twisted at the point where he doesn't move. I don't want him to suffer. If I have to mercy kill him, I will, but I would much rather prefer that he heal. Does anyone have any advice?