A heavy infestation of mites will kill a bird, not suddenly, but slowly.
You can find Permethrin based poultry dust and spray at feed stores. Get some and treat him asap.
Put the dust in a sock, then use it like a powder puff to dust him all over making sure to get the dust through the feathers and down to the skin. Get around his vent, his tail, under the wings, etc. You want those critters gone! Re-dust him in 5 day intervals.
Clean out your coop bedding and nesting boxes. Spray all that down with spray. Remember to treat the roosts too. Repeat treatment in 5 days. You need to treat all the rest of your flock too.
It's best to provide a nutritionally balanced poultry feed. Scratch is usually quite low in protein and not a balanced as crumbles or pellets. Aim for a minimum of 16% protein, but I've found my birds usually do better with 18-20%. If you have a mixed flock of different ages and genders, then an all flock type feed works very well.
I'd continue to monitor his crop. Do provide grit (crushed granite) free choice.
The mites and low protein are dragging him down really bad. Get some poultry vitamins into if you have them. Work on hydration then see that he's eating. Some egg, meat or fish can be given to help give him a boost.
The feathers may all have come out because of the mites, hard to know. But if you want him to be healthy and feather in nicely, then you'll need to make some adjustments to nutrition and get rid of the bugs.
