There was a neglected hatchling I adopted that sounds a lot like yours. He was very small, and he slept constantly. His shell almost felt like paper.
I put him under a very warm light in a small tupperware container with just enough water in there to keep him wet. I also had an adhesive heater on the bottom of the bowl. He wouldn't eat turtle pellets at all. I don't think he knew how. I still floated pellets in the water knowing that he would drink the water and ingest some of the dissolving pellet. How I got him to eat: I found the smallest fish I could (turned out to be guppies; I got lucky and showed up at the pet store right when one gave birth). The fish were about the size of a pencil lead. I put a couple in there, hoping that the movement would stimulate him to eat. Next time I came back, I saw that he'd taken one bite out of the center of the fish. And that's all it took. He started eating more right away and recovered enough to be put in with my bigger turtle within two weeks.
I also stimulated him enough to wake up and move around a bit a few times during the day.
So, I guess all I can recommend is warmth, a small amount of standing water (small enough that there's no threat of him tipping over and not being able to right himself), and encouragement to feed.
If I think if more, I'll post.
Devin