I hope he recovers.
I'd address the lice for sure and if he's that new to you, getting a fecal float would be a good idea to rule out Coccidiosis especially. He's basically on fairly new ground so he may not have resistance to the Coccidia that's found in the coop/what your chickens are...
Photos of his poop?
How new to you is he?
Can you take a sample of poop to your vet to see if Coccidiosis and/or worms are part of the problem?
Hard to know what's causing the symptoms, it seems they are getting worse over the course of a few days.
I'd work on hydration, keep him eating and...