Hi Galaciel21,
I haven't been on the site in a few days and am only now reading about your difficulty with you little guy. I wanted to give at least a few words of encouragement - I had an Equadorian crab that came up from a molt with only three legs, no pincers. I put him in a container and yes for several days I worried because he was barely moving. But I put tiny containers of ocean salt water and fresh water in with him (plastic bottle caps) and he would drain the little dishes. He had about a 6x4" area to move around in. He sat in the fresh water bowl for days, worried the heck out of me, with his shell upended and his little body poking out the top - then he began destroying the food in the food dish I put in there. I was having to refill it every day, because he'd wipe out what I'd put in. I gave him seaweed daily as well as shrimp I had ground up in my blender (that is NOT an easy feat) because he had no pincers to tear anything with and had to depend on his pointy little legs to pick up food, or put his face right in it. So I made sure all his food was mushed up really fine. I gave him protein and chitin sources daily, seaweed (I am a firm believer in daily seaweed!) and bananas and strawberries mushed up. Anything I could mush (sweet potatoes, cooked brocolli, etc) I'd give him. After a week I was able to place him in an iso tank, and he went down for a molt just 3 weeks after he'd come up. He was down 3 weeks, and came up with two adorable undersized claws and a new leg. After another molt another couple months later, his claws were normal size and he was HUGE. So he survived well.
Unfortunately, your little guy doesn't have his shell, and that would worry me a lot too.
I had one that went shell-less, and I put him into a small container full of clean shells, holes up, with some ocean water spritzed in them. He was on top of the shells, and would have had to climb over them to get to a spot that didn't have any, and he ended up taking one. I think that this long without a shell I might consider placing yours in a situation like that, if you feel it might be okay and that he isn't too stressed.
I'd wait for Marie to chime in on this one too. It's just a thought, and your situation is different.
I have to say though, his abdomen, to me, looks pretty healthy, so there's hope!
Edit: About the large pincer not moving as you've stated a few times: If he does make it through this, it may be that his large pincer is useless for grabbing. It is unusual for it to be open like that. You will just have to make sure to give food that is well ground so he can easily pick it up with his small pincer until he molts and grows a new big pincer. He may end up dropping the big pincer too, so don't be too alarmed if that happens.