Is there a way to get a video of him? Upload to youtube or similar platform and provide a link.
It's good that some of the crust is starting to break away, photos of the poop would be good.
You treated for worms and coccidiosis a couple of months ago - what did you use?
If he's having a hard time breathing and you feel he has a respiratory infection, then I would give him the Tylan.
Tylan50 dosing is .25ml per pound of weight given orally 3 times a day for 5 days. If you have Tylosin powder, that's 1tsp per gallon of water for 5 days. If he's not drinking well, then you will want to syringe the medicated water into him several times a day.
I would re-check to make sure his crop is emptying overnight, look inside his beak to make sure there's no canker or lesions. Check him well for lice/mites. I'm sure you've done all this, but give him a going over. Wherever you are keeping him, plenty of ventilation, but also sheltered so he's warm.
He may be enticed with a bit of buttermilk and white rice along with some wet feed. Bits of scrambled egg, etc.
I'm sorry he's not improving.